I have an 95 SL1 when i run the A/C while the car is in park or neutral the car sounds just fine but when i put the trans in a drive gear there is a loud knocking noise until i give it gas then it stops so while i am driving it runs and sounds great but when i stop and idle in drive with the a/c running it knocks again. so i don't if its a motor mount or something please let me know if you have idea on what to check.

 

You go out to your car and look things over. Search on what a top mount looks like when it's bad. Search how to tell if you have a bad tensioner. Come back when you have some an idea what your talking about.

Fair enough?

 

Does it do this with the a/c off? If not, I think you know what the problem is.

 

It does it only with the ac on when I have tabs in a drive gear at idle

 

Any way you could post a video with a sound clip? I'm thinking there's a problem with your a/c clutch, but I'm not really a mechanic.

 

What does this mean?

"I have tabs in a drive gear at idle"

Whats a tab?

 

That was mistype I only get the noise when the a/c is on while the car is in a drive gear at idle. I changed the upper and lower engine mounts and the tensior is good and yes there is enough oil. So I'm still at a lost

 

O.K., just thinking. You changed the upper and lower engine mount. I guess it is my curiosity to ask why. Was there something wrong with them. Were they broken? Was the noise coming from them. I realize my question may sound redundant but I try to work and suggest from a view point that says if it ain't broke don't fix it until it is. Also if you were changing engine mounts just because it seemed like a good idea there are more of them. And should the engine be bouncing around whacking something to make the noise that bouncing should also be somewhat visible indicating that changing an engine mount should have done some good. I have seen bad mounts allow an engine to move and hit something but movement should be visible. And if you really do not see much in the way of movement it probably is not the problem. The noise you are describing does in deed sound like a bad tensioner. However if you can not trace the noise to it, then the only other option is to shot gun replace it too. Why with the A/C on? Most probably because that is added load on the drive belt and something in that system may be iffy at this point in time or the knock actually is coming from the A/C compressor or the clutch.
You need to be able to noise trace. I have found a long screw driver to be handy for this. You place the large handle of the screw driver right on/into your ear lobe and then place the pointy end on or near things that might be noisy. It is amazing the things you can hear this way. The closer you get to the source of the noise the louder you will hear it. Doing this of course has a bit of a hazard attached to it should you shove the pointy end on something that is moving it could get dicy so be careful. But doing this also is an invaluable method of tracing noises down.

 

I just use a funnel with a piece of heater hose attached to it. Lots safer, because if you hit a moving part, it will just yank the hose loose.

 

Hee hee hee -- Yeah, it's really a thrill when you slip a bit, and the end of the screwdriver hits a spark plug ...........