I am new to this forum so bear with me (late, I know).

I fixed my 5210 amplifier years ago when it had the same popping/rumbling sounds (I needed to get into the schematic and use a meter to do it, so this is not for non-electronic types out there). It turned out that the problem was one of the coupling capacitors - a tantalum type that is polarized - between two stages of the amplifier. It was installed correctly but experiencing a reversed bias across it (either a design flaw or the circuitry bias points drifted over time, I do not know which caused it but the prevalence of this sympton points to a design flaw by Marshall). These components can tolerate *small* reverse bias for some time but eventually will fail like we have all seen. The fix was to replace it with a bi-polar capacitor of approximately the same value (the value is not terribly critical here, a larger one will permit more low frequencies through and a smaller one will start to reduce the bass response if too small).

I did this mod to mine, and the amplifier has been free of these pops and rumbles for almost 10 years now.