The Mellotron Group

Every now and again I notice a electrical poping sound when I play a note on my tron, it seems to be completely random, in fact I think it can happen when I am not playing a note. I notice that it is present in quite a few of the M-tron samples so it isn't just my tron. I am curious as to the cause but not probably bothered enough to fix it, especially if it involves spending lots of money or taking the machine to bits.

Any ideas?

Mark