Buzzing on Tubemeister 36

Hi guys, yesterday I got a H&K 36w combo amp second hand. Today I went to test in the room but I was quite disappointed: already on 40% gain on the clean channel I can clearly hear some farting/buzzing noise when I play with a hard attack. It gets worse when I use some booster or low gain pedals like the tchula or the cranker. The sound gets quite farty quite fast. On high gain I can’t hear much but maybe its because it’s covered by the distortion. Using the crunch or the lead channel seem to hide the problem too. If I set it on 5W and I crank the gain up (clean channel still) it’s pretty much a full crunch sound. Is it normal? I didn’t have time yet to plug the DI behind to be sure it’s not the cone. What do you think it could be? I know it’s a bit hard to undertand without hearing it… I recorded something with my phone, maybe I can post that Thank you