Exhaust
 

My 84 300 SD is making a strange noise.
I removed the crossover resonator and replaced it with a straight piece of exhaust tubing. My noise started at about the same time.
I can't put the resonator back on but I don't know what else might be causing the noise .
The car doesn't have to be moving and the noise will come on at about 1000 rpm and go away by about 1500.
Can anyone tell me what the resonator is supposed to be doing.
I can't believe MB put it there for no reason and it must cost a ton. The exhaust is not loud so it doesn't seem noise was the reason.
Thanks,

What kind of strange noise?

What was your car originally equipped with, the dual 'kidney' resonators, or a single resonator?

It has never been unusual for exhaust 'mods' on the turbodiesels to result in resonance or vibration in the cabin at certain rpms. The original equipment exhaust is designed a certain way for a reason.

Did you weld the new, replacement piece in or did you clamp it?

No the OEM setup does not cost a 'ton', but that depends on what your definition of a ton may be....

The noise is somewhat like a rhythmic knocking noise but it is not a metallic, solid sounding noise.
It happens at low engine speed and seems to increase in frequency then disappear with higher engine speed. It is most easy to hear from inside the cabin. It can be heard moving or stopped.
The resonator is the dual crossover resonator which I replaced with a straight piece of tubing welded in place.
I have heard most common engine noises, rod, main bearing, piston etc. and this does not sound like any of those I have ever heard.

X2 compared with Japanese these parts are very cheap.

As for the noise - have a look at the underside of the car with some trustworthy person revving the engine for you.

It's been a while since I had an SD but I seem to recall that once, in a front pipe replacement, the shop broke the first exhaust support which if my memory serves is around where the front pipe curves to run under the firewall.

Anyway, the noise that resulted was similar to what you are describing. Your support may be missing or damaged and the exhaust work you did is just amplifying the problem.

It's most likely a resonance from the engine. When my exhaust finally fell off, I had the nastiest drone that drove me nuts. I sounded like a boat with the farting exhaust.

The exhaust shop I took my car to laughed when I said I took off the exhaust myself. I ended up having the last laugh when they saw that I did correctly removed the exhaust myself. :cool:

Rhythmic knocking: Very likely the exhaust moving around and making contact with something else under the car. Check your hangers and mounts.

I had a similar sound come from my 300D approximately halfway between my origin (Nevada) and destination (Georgia) on a cross country trip. I was a bit worried until I looked and realized I just needed some new muffler hangers.

Not sure if your year has this, but just down from the exhaust manifold is a 8" long secton of braided steel with flexible pipe under it. This area is prone to cracking and an exhaust leak. Mine is leaking now and hear the noise from both in and outside of the car.

Let us know once you find out

Well I've checked everything I can find or think of with no luck.
The exhaust downpipe was replaced prior to all this happening.
The support for that bolts up to the front of the transmission housing with rubber grommets and it all looks good.
I guess when whatever is causing it gets bad enough I will find it.
I don't know what else to check at this point. I am either looking for the wrong thing or missing whatever is causing this.
Thanks for all the suggestions.

This is going to sound cray, but I am wondering if it is coming from the fuel return line going to the tank. Does it have the cigar hose or has someone replaced the return line with a piece of regular fuel hose? It will resonate like hell and gives the characteristics you describe. Mine did it and drove me nuts until I replaced it.