I've had the same issue. I haven't been able to fix it and it comes and goes. In me case it seems to be that something is loose inside. When I've been at my techs he reattaches one of the wires to one of the big steel cans inside, one of the capacitors and that seems to do it. I play 3-4 gigs and it comes back, and goes away in 5 minutes after I turn it off/on but creeps out again randomly 3 4 weeks later and goes away. I've fixed this issue twice the same way.
Proudly using Musicman basses, vintage ibanez copies, and custom builds.
Amps: Ampeg b15n + Acoustic 370
Cabs: mesa 1000 + Ampeg Heritage 410
I stomp on EBS, EHX, and MXR pedals.