What does this sound like?

Exhaust would be my suspicion , loose bracket or shield
Ron

 

My first guess is that its the ACC belts - smacking down. Spray them with some belt conditioner while its running - DO NOT USE WD40... or any lubricant. if it goes away then its time for changing the ACC belts.

if the car sits for a while the belts take on a memory and they slap a bit at first.

if that does not go away, I would then not run the car and check that your valves are in time, and that there is nothing in the valve covers & belts are good.

also could be the AC compressor.. but doubt that.

 

Go to Pep Boys of any auto store and buy one of those noise probes with a headset. Then put the probe on the alternator for a starting point. I had a noisy bearing in mine. If not check around the exhaust and keep going until you locate the source. I sprayed belts on one of my cars and it was kind of sticky spray. That noise turned out to be the idler hydraulic tensioner and spring.

 

The erratic nature points to the ac belt slap, although this sounds slightly more metallic. Does it go away or change when you turn on the ac compressor?

 

I would check the clearance between the alternator and the water pipe that goes from thermostat housing down past alternator and around front of engine.

 

If it is not the AC belt, then it could be the water pump axle. On my 328 the axle was worn, causing the impeller to contact the engine block inside (rattle).