It is either the moose or the radiator fans (high speed) kicking on. DEF and DEF/FLR causes the A/C to come on (even when it is turned to heat) to assist in drying out the air. And moose usually shows up with the A/C on.

Read this thread and link to moosing sound.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=v6maint&Number=1148660&page=&view=&sb=5&o=&fpart=all&vc=1

"Always do the cheap and easy ones first." 1996 V6 ATX 96K miles