New motor mounts = big improvement, but...roaring sound
 

I took my '87 300D into the shop for new motor mounts, lower ball joints, and a general check-over. After the job was done, it is like a whole new car. So smooth! My old motor mounts must have been really shot.

But the wierd thing is, now the engine sounds different. There is a "roaring" sound that increases with the RPMs. It sounds a little like a 4wd vehicle in low gear. It's definitely the engine though, not the tranny. The only thing I know they did to the engine was to charge the A/C, which was previously void of refrigerant. The sound occurs when the A/C is not actively in use, though...

Any ideas?

it could be anumber of things, but i would take it back to the shop where u had the work done and have them take a look....

ive narrowed down my roaring sound finally. its from the exhaust having a loose flange at the exhaust manifold. but i need to get new bolts to get it tight enough.

Fan clutch engaged more often?

Does is do it while moving or while sitting still and pushing on the pedal? If while moving if could be wheel bearing(s).

I had the same problem after the vendor changed the motor mounts out. It was either caused from not loosening the connection before jacking up one side of the engine or perhaps the stress transfered from the proceedure.
A few turns or partial turns shut it up in my case. It also might have been he loosened them up himself to change the motor mounts and forgot to tighten them.
The competence of the three muffler shops I stoped at was interesting. All said they did not have a donut in stock for it. So I drove all the way home as I did not know back then there is no donut on them. Leaving the windows up and heater fan running kept the fumes at bay.

I think now that the roaring is coming from my AC compressor. Charging the system must have put a load on it that was previously absent. Also, there is an electrical short that causes it to run all the time. If I turn on the AC controls in the cabin, then the noise is diminished. My indy tells me that until I get it fixed, I need to run the AC all the time, or I might burn out my evaporator (or was is condensor? - I'm not well-versed in the AC system...)