crackling and whoomph noises!

Listened to a pre amp today (2nd hand) after an initial demo yesterday.....problem was that when the pre was connected to power amp and speakers, there was a very faint rumbling in one speaker only....not really a crackle as such, but often a whoomph sound that interrupted the silence....this then got worse and started happening during cd playback.........its nothing to do with the cables, or power amp, as \i tried more than 1....its not even too bad really, but wouldnt elect to part cash for it, when clearly not right....any suggestions what it may be and how to fix......twas a welborne battery operated amp ( batteries were full capacity, btw) and it was inn good order apart from this...no loose connections upon inspection, but may have missed something.....any thoughts anyone?????

 

Can you give a few more details on the unit ...model number etc

 

You get that kind of sound when electrolytic capacitors go electrically 'leaky' (i.e passing some current). Replacement should effect a cure, but as you say, it's not right.