Threshold SA/1 monoblocks cap issues, perhaps?

Few weeks back I bought these monoblocks and added to my system consisting of Metrum Adagio, EAR 868L, Focal 1038BE and laptop working as a source which will be replaced with a custom made computer based transport in few weeks.

But these monoblocks seems to have few issues though the seller told me that everything was fine with these monsters.

1. There is big thump when I switch on these amps one by one and also within a second I hear a high frequency sound like "ptakk" from both sides. This sound is there while switching off the amp though less frequently and intensity IMO.

2. Initially I thought the sound is only while switching on and off but later I found that it is also there in between while the music is playing. This is more like "karr-karr" like.

3. One day I also heard it even when I switched off amp while rest of system was on. So I am thinking it might be due to leaky caps. All the caps look like in really good condition and seller told me that there is no need to chnage these as these have hardly been used. Even I use these for 20-30 min per day five times week.

4. Once I removed EAR868 from the chain and same issue was still there indicating EAR is not the criminal here. Adagi was working fine with Parasound Halo A21 which I had earlier before I added preamp and monoblocks.

I do not have much knowledge but I can learn quickly. Need the advise of experts as I really love the sound signatures of these monos.

Never left the amps on for more than 30 minutes. The amps are connected to voltage stabilizer and power conditioner and I turn off everything after I am done playing music. I only have about 30 minutes for music 5 days a week.

Amps ere bought at end of this February.

At no point all the wires cross or come close to each other more than their respective inputs/ouputs behind all my equipment.

I have spare Parasound A21 and I prefer the sound of monoblocks.

BTW some renovation work is going on in our home and early next week I would check few internal parts though I doubt I can find a faulty piece.

I know this is an old thread. Does anyone here know the rail DC voltage is on these amps? I am thinking of changing my S/500 to strictly a class A amp and would need to reduce the voltage from the 76vdc it runs at now. 

Thank you