@vic0239
OK on your further testing as per suggestions.
As previously advised, with these results I would have first gone for a "suppressor" as b*cat correctly describes, however, as you have now sourced and fitted a replacement tank temperature stat which has cured the problem, a good result achieved.
It's quite possible that your replacement stat has a suppressor fitted as standard across it's contacts, many now do.
Also apologies to g3uiss for thread hijacking and good luck with your external noise source identification.