Weird VFD behavior - the THUNK?

I've just finished wiring up a 3HP variable frequency drive
(Omron/Yaskawa G5+) to my 1.5 HP Bridgeport. Feed to VFD is 3 phase.

I go from the line to a Corcom line filter to a line reactor to a
wrap around a ferrite core (one wrap) to the VFD to a load reactor to
the BP. There is a braking resistor (ferrite core on brake wires).

Motor is not running in this case (no load condition)

Problem:

Every 15 minutes or so, a sound like a gunshot comes out of the
setup. No smoke, no sparks, LED's on VFD do not even blink - seems
like no harm is being done. This sounds like hitting a garbage can
cover with a baseball bat (or like a VERY large contactor thunking
in).

Since it happens so intermittantly, I haven't been able to pin down
what item is making the noise, but it's scary as hell.

Maybe the line ferrite core should come before the reactor? Could it
be that it's becoming magnitized and thumping against the metal
housing? I'm really confused ;-o

Anyone have a clue?

Jeff