95 Sedan Deville with 4.9. 85K miles. Seems to knock under acceleration. Is this what I'm hearing, or could it be somethiung else?

Are you using premium gas?

If you hear a sort of light rattle at part-throttle acceleration, nothing at wide open throttle acceleration and maybe a quick burst of rattle at upshift, then your EGR valve and system needs cleaning. Choke cleaner, hand tools, and a quiet morning. (Nothing you can pour in the tank will clean an EGR - it takes hands-on DIY.)

If you have a steady, hard rattle or knock on acceleration or uphill loads, then it's gas, timing, or engine mechanical problems.

If you have a loud, hard knock all the time, mechanical troubles.

If the car is in park and I rev the engine, it doesn't seem to make the noise. When it is in drive and under load is when I hear it. It isn't really a loud pronounced metal knock, more of a muffled thumping.

A "muffled thumping" appears to be something mechanical. Spark knock or EGR knock is more like a hammer ringing a big block of steel or the "peas rattling in a can" noise.

Exhaust system hangers, engine mounts, FWD cradle mounting ?

Haven't heard anyone complain about this lately, but the classic main bearing knock can be a possibility. The 4.9 has a tendency to gain too much clearance on the #1 due to distortion of the aluminum block around the main bearing bore. A service main bearing was available that was oversized to fix the issue. The symptoms were the same as you describe. However, the only real way to check for this would be to pull the pan and physically check the clearance.

As previously stated, be sure this isn't simple spark knock. Make sure you are using the correct fuel and the fuel system is working properly, particularly the fuel pressure reguator. Make sure the EGR system is clean and functioning properly. The correct spark plugs with a correct heat range should be used. Timing is computer controlled, but be sure the initial timing is correct. It's done like any regular small block chevrolet, except you have to make sure to jump the A and B terminals on the diagnostic connector to put the car into "set timing mode". Loosen distributor hold-down bolt and run engine at any speed under 800 rpm. Timing should be 10 degrees BTDC.

I think the main bearing thump is usually heard at idle if I remember correctly.

Look down the TB. See the two EGR tubes staring back at you? Are they carboned closed? If so, rod them out and your problem will be solved.

The usual crank "thump" is most noticeable at idle, but does it at most speeds. My 4.1 is still "thumping " along:bouncy:
When the engine is cold, it's quiet. As it warms up the noise starts.

1: Make sure you're running NO LESS THAN 91 octane fuel

2: Make sure the base timing is at 10 degrees 'before top dead center' (BTDC) with ALDL jumpered

Also note at what operating temperature you hear the sound......