Reducing noise for a TDA8947J amplifier
I've met a noisy low frequency in a portable cassette player with subwoofer a long time ago. The noise wasn't humming or buzzing, but more like a soft crackling noise (static) and the level of the noise was unpredictable, it slowly varied over time and had annoying louder "rushes". The volume or EQ controls did not affect it, so I feel this is a similar case.

I found out that twisting or compressing the heatsink against the PCB reduced or eliminated the noise. I fixed mine by attaching a stiff copper wire hook pulling the heatsink against the PCB. At least you can try if applying force onto the heatsink makes a difference.

I wasn't able to get into the root cause, I can only make guesses:

1) Chip fault, assuming that the applied external mechanical force enhances the internal connections.

2) Electromagnetic loop/interference with the currents of the chip and the aluminium heatsink -> make sure the heatsink is properly grounded.

I recommend to resolder the heatsink and chip in any case. Heatsink soldering can break due to temperature changes. The fracture circle around the "leg" can be seen in proper light, if at all. They're often nearly invisible to the naked eye.