Problem with Boss BD-2

[mudmen]

Few days ago I bought used Boss BD-2. It's a really nice sounding pedal but I have one small problem...

When I hit the strings harder apart from slightly distorted tone I can hear some strange buzzy noise. It appears only for a little while during the sustain...

I tried to change batteries but the hum didn't disappear...
I have no clue what to do

I can post sound clips to let you hear what's going on...