buszzing only when both tones maxed (hollowbody electric geetar)

[K-Frog]

Hey guys,

I'm not sure where this noise is coming from.

Specifically, I'm working on an Oscar Schmidt OE40B hollowbody electric guitar.

humbuckers, 2 vol, 2 tone, 3 way switch.

It buzzes when:
1) on neck pickup and tone is maxed
2) on bridge pickup and tone is maxed
3) both pickups and both tones maxed (does not buzz when one is maxed and one is zeroed)

Does not have a bridge ground wire that i can tell (bridge saddle and tailpiece)

Seems like it should be something simple, but I'm not seeing it

Thanks in advance