After many years amp. began making persistent scratching noise, mostly in R ch. but also to a lesser degree in the L ch. I noticed that hitting the front of the amp with my hand even lightly, first causes an instant increase in the noise, which fades quickly & sometimes stops briefly, or just becomes less. I don't really use the amp because of this. Possible loose wire, faulty solder connection, or bad component? Any help would be appreciated. Thxs.