I once heard a pop that almost made me lose a load... ... it turned out that the breather hole for the crankcase on the pump still had the factory plug and when enough pressure built up on it's first full run it poped out with a noise like you describe.
Not sure of the brand but could be the same problem...
After finding the vented cover in with the instructions and installing the breather everything was fine.
Last edited by milo; 05-13-2006 at 04:50 PM.
Reason: can't spell