Strange ticking noise when amps were powered on from cold

Any one encountered this phenomenon? I have a pair of Aesthetix Atlas Monos. When they were powered on from cold I could hear some continuous ticking noise (10-15 beat per second) if I stick my ear right next to the speakers. VERY faint. The noise was gone in about 5 mins once the monos were warmed up. As long as the monos’s main switches were on the noise would not return regardless the monos were playing or stand by. It only happened when they were turned on from cold (e.g. after a power failure in the neighbourhood ). searched online and some people suggest that it is the capacitors charging?

Steps I’ve done to diagnose the problem:

1. Turn off preamp-same problem

3. Replace 6SN7 tubes

3. use other amps-no noise

4. plug them in other main sockets-same problem

5. switch off all other appliances at home (fridges, dishwasher, modem, router, down lights)-same problem

6. Disconnect all rechargeable devices (laptops, robot vacuum cleaner, power bank, electric toothbrush)-seemed to quiet the noise a bit but not to sure as it was very faint in the first place

At the moment it’s not very practical for me to take the monos to other places to try.

Your input is much appreciated.