99 SL2 chattering up a storm
 

New to the forum and I have been looking around trying to figure out what exactly my car is doing. I have a 99 SL2 1.9l manual transmission with 186k on her. About six thousand miles ago it started to make a very feint chattering sound around 40 miles an hour, but it would go away around 50 mph. I didn't have a garage at the time and I had planned to trade the car in soon, so I didn't worry about it. Do to circumstances, I am keeping the car a while longer, so I need to get this figured out. It has gotten worse in the past couple of weeks. It starts at 35mph and the chattering speeds up as the car gets faster, but 35 to 45 mph is when it is the loudest. It is coming from the driver side of the car and when I cut the car to the right it stops making the sound. I also noticed tonight if I am going down hill and I let off the gas it isn't as loud as it would be going on a level hill, but if I put the car in neutral, clutch or no clutch, it returns to the normal chatter sound. And when I say chatter, it sounds like a couple of stones in a tin can that are being spun inside the can. Today I tried to reproduce the sound with the car on jack stands in my new garage (yay! somewhere to work on things). It didn't make a single sound. I tried it in 4th and 5th gear at speeds ranging from 35mph to 45mph with both hubs spinning. I didn't try it with the wheels on. The cv joints appear to be solid, the rubber looks great. There is no play in the axles other than a slight bit of play on the drivers side output on the transmission. The wheel bearings are not making noise nor do they feel grindy or loose. So the only thing I can think of is something is going in the transmission when both tires get to spin off the jack stand. If anyone has any idea where to look I would appreciate any input!

Take your hubcaps off and go for a drive, thats if you have hubcaps.

maybe a bad motor mount?

Any chance it's brake related? Maybe a stone stuck in the pad or a bent backing plate?

Maybe reply?

i know you said driver's side but how about the heat shield on the bottom of the cat?

Sorry it took a bit to reply, busy with work and moving. Car has alloy wheels, so no hubcap. I do think it needs a new motor mount, but the car still makes the noise with the engine shutoff. It is not the exhaust as it is solid and nothing makes a sound if I shake the exhaust. It has really got me baffled. I guess the only way to figure it out is to start replacing stuff.

I know I'm chiming in well after the first set of comments, but I would look at the front spoiler/air dam, and the plastic grill mounts. Perhaps, the air blowing against the front of the car is causing these things to flutter/vibrate, making the noises you hear ...

I'm going to have to go back to the beginning. While I have chased a lot of strange noises and a loud chattering sound stopped me from going to L.A. on on trip which turned out to be a piece of chrome time coming loose from the windshield in the wind that took me two days to figure out and did not see it until the sun was just right and i saw it with a shadow on the ground while driving the car. It still sounds like a CV joint. Which will do different things when the car's weight is on the wheels then when they are sitting free with the car on jack stands.