Would be a darn fine place to start. Stuck EGR valve can sure mess with the air/fuel mixture ratio which will definitely affect the idle speed and performance...

I don't think it would cause your knocking however ....

 

O.k., now you got a code. One of hundreds that are available to see what it going on. The code defined is slightly different than stuck which can mean a variety of interpretations.
It means the EGR valve is not closed.
Another way of saying it is open.

what does that do?
In old school language it is a controlled vacuum leak.
And more or less causes problems a vacuum leak causes only specifically it functions using exhaust gases and uses them to reduce certain forms of emissions. The bad guy directly related to engine performance. But you don't have the problem of emissions, you have the problem of a high uncontrolled idle, a symptom of a vacuum leak.

Dah-da!
Now you know what to fix.
Probably locating and cleaning it because it is full of icky carbon (yes a high tech term) causing it to stick.
But what else is going on.
You have a knocking noise atg a certain engine rpm.
Now we need to define the word
knocking.
And I can not do that from where I am sitting.
It is or might be a mechanical noise and common ones are a bad bearing, piston pin knocking, piston slap, something in the valve train or another one which has nothing to do with a mechanical problem.
Something called spark knock or pinging. A rattly noise most of the time I have heard it, mostly defined as uncontrolled flame front when the mixture in the cylinder is firing, a mini random explosion inside the engine. First mostly a problem with the early smog controlled engines caused by poor mixture and using gasoline that was not the proper octane rating for the engine design or current adjustments and still is, although the advent of computer control re-adjusts things so crap gas, the 89 octane stuff will actually run with out causing spark knock or pinging as it did with engines that did not have computer monitoring and operation. That and some advanced mechanical engineering.
But the point is?
How do you define knocking?
The some what heavy mechanical rap of two pieces of metal whacking one another making the noise due to poor lubrication or worn parts, or the sharper knocking sound make by spark knock or pinging that can be caused by a vacuum leak or a stuck EGR valve.
I can tell the difference! Can you?
Fix the Egr Valve if you can't tell the difference and see if the knocking goes away.

 

"Knock" might be a strong word for what I am hearing. "Click" is much to weak of a word, though. It's a heavy click, I suppose. You can't feel it, but you can hear it. As for the EGR, it was definitely replaced very recently; it is shiny-new. The PCV was replaced recently, too. The dealer I bought it from said that the knocking noise wasn't there when he picked up the car, so he took off $500 from the price. I suspect that the EGR was replaced along with the PCV to try fix the problem, and it did fix it--for a short while--but after time, something messed up the EGR and started the problem again. Is there something that could mess up a new EGR in a relatively short amount of time like this? Or is my whole theory bogus?

 

The EGR valve may be just fine, but the passage on the intake where it's connected could be full of carbon. I'd remove the EGR valve and see if the passage is clear or clogged. Get some stiff wire and run it in there to clear it out.

 

Once again Ruby has the right idea. A new piece of data is now been learned. The
egr valve as far as you can tell is new and it might be. And he EGR passages might be full of icky carbon, did you look?
Of course if neither of us (Ruby and I or anyone else who wants to step in) have ever pulled the EGR valve on a Saturn neither of us have a clue what they are supposed to look like when they are clean.
But the very nature of the beast, an EGR valve also known as an Exhaust Gas Recirculating or something like that name implies that exhaust gasses pass through it and Exhaust Gasses by their very nature are full of icky goopy stuff. And it is not surprising that it might plug the valve, it's passages and result in poor running. So now what might need to be defined is what the word fix means.
My best guess is fix means, clean or replace or some combination of those two items. What can cause the EGR valve to go bad if it is new?
I am not sure you have identified that it has gone bad.
It sounds as if the word clean has not been applied.
A stiff wire as Ruby said, personally I like the concept of carburetor cleaner in a spray can. A liberal dose of Carburetor or fuel injection cleaner and poking around with a stiff something might just go a long way towards cleaning. Just in case it happens to be dirty. Spray Cleaner and some compressed air would help. Do you have compressed air?
If not, you can buy the stuff in a spray can at an electronics out let and it is used to clean keyboards with.
Me? I have plenty of carburetor cleaner and if I want compressed air, I turn on my compressor.
Now carburetor cleaner can pretty much rot your sox. So spraying it on something that is painted just might not be the smartest move.
And just in case you start thinking oh no! I don't want something to rot my sox so I will buy brake cleaner in a spray can. Forget about it. Brake cleaner does not clean anything. I am not sure what exactly it is supposed to do other than cost money. You do want something to rot your sox. Carbon build up needs to be dissolved. A liberal dose of it and compressed air to blow gunk around and away goes a long way towards cleaning. And try to use a bright light to see what is being cleaned.
Good luck.

 

I appreciate the experience this forum has. I took my car down to my mechanic, and he was pretty sure that I has blown a piston skirt. I haven't been the best at describing my situation. Maybe some of you would come to the same conclusion if you could see the car for yourself. He said that I might be able to get some more life out of my car if I grandma-ed it, but if I tried doing that, the engine couldn't be re-built because of damage to the cylinder wall. I was a little too wooed by the appearance and should have been more focused on mechanics--a beginner's mistake.

 

I've been pulling my own wrenches for over 40 years and have never heard of such a thing. I'm not saying it can't happen, I just don't see how that could be determined without pulling the oil pan and having a look at each piston and cylinder. Did he do this? If not, I'm curious as to how the hell he came to that conclusion. I'm sure some of the other guys here are too. Can you enlighten us?

 

Gotta go along with Ruby again. Gotta watch that, if I associate to much with him he is gonna get a bad reputation. Now I am older than he is so add a few years to his statement. And you have no clue as to the condition of some of the engine I have rebuilt. And one of my basic rules in self maintenance is simply that I can screw things up cheaper than a professional mechanic so why pay the big bucks when I can do it so much cheaper my self.

Damaged cylinder walls? Broken of damaged piston skirt? Sure I have heard of both. Never experienced either but I can tell you this. with out physically looking at it the diagnostic is a guess and it is not a very good one.
A compression check will tell that there is a cylinder in trouble. Was that done? If not, then no one knows. But it can not tell why it is in trouble. Although there is a way to sort of zero in on whether it is valves, which has nothing to do with cylinder walls and rings, which does. Although a physical inspection is in order. And that can be done with limitations usint a bore scope or some other way to insert a light and a digital camera thing through a spark plug hole and look. And this is not perfect but it is a diagnostic tool. Was this done?
Or did the well qualified individual stand there looking at the shiny paint and scratch his nuts and say, Well, it's gonna cost you money so no sense in getting a second opinion because no one around here is better than I am?
I have put a lot of high speed miles on a much modified engine that was pulled out of car that should have been crushed and was, the engine was so crappy hot tanking would not clean it up. It was put into a machine that worked like an oven cleaner and heated the begeezus out of it untill the crap in it burned off. And then I rebuilt it.
And a cylinder can be sleeved and has been when some one puts a rod through the side of it leaving a big hole in it it into a water jacket.
At some point in time to fix it all it takes is money.

And you are not even close to that based on the information in your posts so far.
It sounds like you bought a nice car. Cosmetics are the selling point. Now all it takes is to find out what is wrong with it and the shine on the paint does not indicate anything about that.

 

Spidy -- I'm reminded about that commercial that's running on television right now... A bunch of buys are standing around a grill shooting the breeze about the exploits of some unnamed sports jock, and the first guy makes a claim and says, ..."I'm 99.9% sure of that." The second guy takes a bite out his burger, chews for a couple of seconds, swallows and says, ..."So you really don't know"...

Without eyeballs on the problem, you really don't know

 

Spidy, what part of the country do you live in? Maybe there's a member here that's close enough to help you out. Sounds to me like someone's trying to lighten your wallet and make theirs fatter!