Hi everyone, I have had this Deluxe Reverb for a few months. I received it in a trade knowing that it had already been modded to hell and back but it's always been reliable and I got it for a good deal. About a week ago my amp started making a loud muffle/ humming sound. I quickly turned it off, let it sit for a while and then checked that all of the tubes were seated properly, they were. Turning the amp back on it seemed to be back to normal, so I played a while longer, turned it off and went to bed. A few days later I was playing when the noise came back, I gently tapped on top of the head and the noise went away again. I can't replicate this issue, it more so just happens randomly. Turning the amp off and tapping on the head makes it go way (silly I know). It is a 1990 Deluxe Reverb if that makes any difference. I know likely I will have to take this amp into a tech to get fixed, but I am not super familiar with electronics like this and was wondering if anyone could share any knowledge on a potential DIY fix or ideas on the issue. Thank you! Please note the intensity knob on this DR is set to be a master volume.