I think it could be just an exhaust leak. When these cars leak at the manifold, it's a TACK TACK TACK sound, like a bad valve. The gaskets rust away and burn out. Often you can see a bit of the gasket metal sticking out around the flange. They may be quieter at idle because of less airflow- accelerating would be loudest, with the throttle open more.

Also, it's not at all uncommon for cars to spit water from the tailpipe- water vapor is a normal byproduct of combustion. If it's cool weather that forms condensation. Mine makes a puddle if left to warm up idling for a long period.

The other items may be just perception because you're hearing the noise and just being more aware of the engine. Or it could be a real effect due to the leak. Either way, try to rule that out before getting involved in major engine work.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 234K, '82 245T/M46-182K, '84 242DL/AW70-100K, '89 244DL/AW70- 212K and I miss my: 86 244DL 215K, 87 244DL 239K, 88 744GLE 233K, 88 244GL 147K, 91 244 183K