Sunfire True Subwoofer pops

[Drdave]

We have a Sunfire subwoofer in for service. After replacement of many capacitors, the unit seems to work fine except for an occasional loud pop or thud from a normal audio source ,but with a steady low freq. test tone it work without any pops .Any ideas? Someone said it maybe switching intermittently in and out of standby on normal audio.We don't have any service info on this so it makes life even harder.
As always any help would be appreciated.