hello everyone, i got this used fender twin amp today and it sounds great, but there is a weird sound happening, sounds like something is winding up, it will keep going than stop, than do it again, repeats constantly. i figured its the reverb because it gets louder when i turn it up and vanishes when turned of, it also gets louder when you turn the amp up itself.

does anyone have experience with this or anything like it? i need to know because the amp is under warranty, bought it from a shop.