strange engine noise

[aggie8387]

I have a '95 Blazer that sometimes makes a strange noise when accelerating from a stop. It's a thumping, hollow type of sound, kinda like hitting the hood with your knuckles. It's not knocking or pinging. It only does this once in a while when the engine is running rough. It will make 2-3 thumps and then quit until I stop and start up again. Also when it does this it always hesitates/has no power for a few seconds.

Any ideas on what could cause this?

[BlazerLT]

Any check engine lights?