subwoofer only works at higher rpms

I own a 72 Chevy C20 truck. I recently installed a post head unit from custom auto sound in an effort to keep a vintage looking interior, along with two pioneer 6.5s and a dual voice coil 4X10. Those components by themselves work fine. Last night I added a Kenwood subwoofer/amp unit consisting of two 6.5” subs in a small factory box and a built in 150W amp. When the truck is idling the subs work intermittently. Each time the bass hits it’s like a switch is flipped on and off during the speakers oscillation. When the engine is not running and the key is turned on the subs sound great. Also when engine RPMs are increased (highway speeds) the unit sounds great. The truck has a new 55 amp alternator which is more than the factory 42 amp that came with the truck. Thanks in advance for all the help.

Have you checked for sub polarity and/or engine "noise" interference?