First time w/ Tubes - pings/noises normal?

I've been a very happy owner of hybrid tube/SS amp and pre from Vincent - SP20 Amp, and SA-T7. I have these currently paired w/ Rega RX-3's, which I like, but need more from a speaker; and a REL S5 sub, which is amazing. Music is coming from a Cambridge audio 851n network streamer.

Anyway, I love the warmth and tone of the sound i'm getting. After a few months of daily listening, i noticed that as the pre starts up, one of the channels will ping, and sometimes there's a static 'air' sound that can be heard. Then it goes away. Sometimes it will do a ping mid-listening. I had never heard this before, and assumed it was defect. I narrowed it to the preamp, as when I removed that from they system, it was all gone. I've also switched up all the cables, etc...  I sent it back for repair. The shop couldn't find a thing wrong, bench tested for a week, etc.. Couldn't reproduce what I heard somewhat regularly. They sent it back to me.. and it was still there.  I sent it  back again - and though they could not reproduce, they agreed to swap for a new one.

So i install the new pre, and 2 or 3 months of use, not a single ping. Then suddenly a few months in, starting to do the same thing.

I'm now wondering - is this just what happens to tube gear after it breaks in? Or something similar? Is there something else i'm missing here?

Thanks!