The thump thump heart beat

I have a 97 explorer 2 dr. for some reason when I come to a stop or turn the engine off I get this thumping noise that sounds like it is coming from just ahead of the passenger compartment --it starts out fairly loud, at about 1 sec intervals and slowly decreases in intensity and usually fades with in about 10 - 15 sec. At first I thought it might be liquid lock in the cooling system -- but the cap press checks fine -- what could make that kind of thumping noise ?? any suggestions??

Flushing the cooling sysem and turning the thermostat over ( someone before me didn't even know how to install a t-stat) helped a lot, but still occasionally occurs-- there is a Ford service bulletin out there ( # 12583) that addresses a "thumping noise from the cooling system.....", I have yet to find it online..