Pass labs X250.5 cracking noise

Hi all, recently bought a used X250.5 from a dealer on audiogon. Really happy with the SQ. The amp makes a crackling noise after powering down. Its a few cracks/pops - very intermittent and then it subsides. Spoke with Pass engineer and he assured me that the noise was coming from thermal contraction of heat sinks etc after normal operation. This weekend I noticed that the amp made cracking noise during warm up too. Again this is one or two and then after the amp warms up - i dont hear any more pops. Or i guess the sound from speaker masks it perhaps? I did pause music and noticed the amp for about 30 minutes and no cracking/popping noise during that period. Anything I should be worried about? I'll call Pass again this week - but wanted to see if there were any thoughts?
i have recently bought a pass labs xp.30: connected to 2 different amps i hear a crackling noise from the right speaker. Either on warm up or also later. Then it may totally disappear, but when i thought it was gone (i made some more order in the cables) today i heard it again. When on muting of course the noise cant be heard.
@fabio3181 I would just contact Pass directly. They're great even if you bought decide hand and will help you diagnose and fix. 

Not that I think you need another source for fixing, but the community at is also super knowledge and helpful.
pistha, if what you’re describing is coming directly from the body of the amp and not through the speakers, this is very common with Pass amps. Whoever you spoke to there is correct - noise due to thermal expansion and contraction. Annoying, but a lot of samples do this. 
@tonyptony - i've moved on! the cracking noise from the x250.5 doesnt bother me any more. It only happens during the first 10 mins and then after powering down the amp. @fabio3181's problem seems to be unrelated to thermal expansion - i wouldnt expect the pass preamps to have any measurable thermal contraction/expansion. 
Yes the crackling or popping of the heat sinks while warming or cooling is completely normal, this is not heard over the speakers. You should not be getting noise from your pre my XP 10 is dead silent.