Sounds like it might be tube rattle, I was reading the reviews on the thomann site for the tube 15, and someone else had this problem.
----------------------------------------------
"" great value
eoinc 05.05.2021
ok - there's no escaping the price here. this would be fantastic value at twice the price.
Pros:
its a great sounding, very versatile amp. the 1W setting is perfect for getting some crunch at home volumes. At full power its loud enough to gig with, although you may wish for more headroom.
very useable SPRING reverb.
Speaker gets a bad rep, but is actually fine once you break it in.
power tubes (surprisingly) were TAD (tube amp doctor).
3-band EQ with 'tone' master control is very nice .
Cosmetically very nicely finished.
Cons:
-cabinet is somewhat thin MDF - fine at the price point
-the cabinet is about as small as it physically could be -
a factor of the 'big speaker/small cabinet' is a tendency for these amps to have tube rattle on first power tube (seems to be a common issue)- this is minor, but annoying, can be fixed by use of tube damper rings (a side note - Thomann were very responsive and helpful with this, so thanks).- no name preamp tubes are only ok - I swapped in some tungsol - improved the sound significantly.
In summary - a 50euro investment on some new pre-amp tubes and
tube vibration damper rings will put this from the great to the amazing "
--------------------------------------
https://www.thomann.de/ie/harley_benton ... eviewlang=