I have a 99 CLK 320 with only 55K miles... Lately I have noticed some sort of a ratteling noise underneath the car every time the car warms up and I have my foot on the brake. The car also seems to have a little viberation when its in idle and my foot is on the brake, however, I don't notice the viberation when the car in Park. Is it time for a fuel injector cleaning? or a tune up? I also notice that there is a hissing sound near the rear right hand wheel, is that part of the muffler or is something come undone underneath the car...

any suggestions would be helpful.....

 

It sounds like the car is missed firing. The spark plugs are good for 100K something if I am not wrong, but consider about all the years, I think you should change them. Also double check your Air filter make sure it’s not clogged, change a new one if needed.

Good luck!

 

Im not sure if the 99's had the 100,000 spark plugs installed. I do know however that even with 100,000 spark plugs, you need to change them every 5 years, whichever comes first.

I had a rattle underneath during acceleration, and it turned out to be the catalytic converter.

 

could be the engine and transmission mounts...never heard of any going out that early though just had mine changed at about 83k mi oin my 99 320

 

That makes no sense, if the car is misfiring, having it D or P would make no difference. It's probaby the transmission mounts of and/or cat converter rattling.

 

It might not be misfiring in this case as some of you claim for the cat converter , but P = No load on the engine -> even with misfiring you will not feel it.

D= Drive -> there is load on the engine -> it will sake more when first step on the gear.

It's not hurt to check

 

I had the same symptoms @ 85K, and it was the cats (although the rough idle was likely a separate issue which was only resolved partially later with motor mounts aand MAF). Good news is that if it is the catalytic converters, you are warrantied until 80K, and it will get much worse long before then. As the cats begin to disintegrate (prematurely usually due to heat, and lots of short trip city driving with frequent stoplight idles), the composite inside will fall apart and rattle around. But under the pressure of higher RPM's or acceleration, those particles are pushed back and can no longer be heard. Heat shield loosening shows similar symptoms, and is an easy fix (but not warrantied). Wait 250 miles, and you'll know if it's cats.