[Gilliam]

Yesterday I was stopped at a light and heard a noise like a big rig was next to me.
No cars anywhere. When I started driving it sounded like the big rig was right beside me.
Then the noise went away.
This morning the car ran normally.
After a while that noise came back. Then it quieted down and then changed to a metallic knocking sound.
Otherwise the car ran fine. I was close to home so pulled in and opened up the hood.
Took a video. What could this be? Is it engine related or water pump or AC pump?

 

[cowboyt]

Wow....that's LOUD!

OK, first thing I would do is, get an automotive stethoscope and touch the end of it to various parts of the engine. You want to find out where it's the loudest.

Was yesterday the first time you heard this noise?

 

[Gilliam]

Yesterday I heard the noise like a big rig following me but not this knocking. This morning car was running without any strange noises and the knocking started when I was returning home. This car has been making lifter noise since I got it but very mild. From googling this it's probably a collapsed lifter.

 

[cowboyt]

Yep, get that automotive stethoscope. I had a mild case of that once upon a time. Replaced the lifters, and it went bye-bye.

 

[Gilliam]

Yep, get that automotive stethoscope. I had a mild case of that once upon a time. Replaced the lifters, and it went bye-bye.

I'm taking the car in next week. What should it cost to replace lifters? Going to an independent mechanic who used to work at a Mercedes dealership. He's getting his business started.
Should I only replace the bad lifter or the whole side?

 

[cowboyt]

That's what the automotive stethoscope will tell you. You want to look for where on the engine that sound is the loudest.

As for replacing lifters, it depends. It can go from reasonable to high. To do so requires removing the rocker arm assembly. I find it akin in difficulty to replacing, say, the conductor plate of the 722.6 transmission or the tranny solenoids. There's some labor involved, of course, but the job's not "hard"; you just need to pay attention, that's all.

 

[Gilliam]

Well it wasn't lifters. It was the A/C compressor pump.
Having it replaced. Pulley was wobbling badly.

 

[cowboyt]

You know, I should've guessed that possibility myself, because the wife's 2000 S500 had a bad pulley. But it's kinda hard to do that sort of diagnostic when you're not right in front of the car. Glad to hear it's not the engine; AC pumps on these cars are not that hard to replace, fortunately.