I have observed the type of problem you quote with Switch mode PSU's.
I think it might be due to shortcomings in the design or old age, that only show up when the input voltage has wandered outside the norm for some reason or other. When powered from 'cold' the starter arangments work OK, but while working (summise) if the mains volts fall, the feedback loop becomes unstable and rather than behaving in the way it was meant to, maintains the output voltage by switching on for long bursts - thus the chuff chuff.
What was really required was to try this device off a variac, pity this was not done. I have seen Saterlitte receivers PSU do this sort of thing (and seen it on the picture), the cure being " new electrolytics" all round. Which brings in another aspect of this thread, whereby , it apears because of finacial considerations that poor grade Electyrolytics in these things are the norm. Have you noticed that computer PSU's last longer than say 'Pace' PSU's. they are cooler and use 'better grade stuff inside'.
Switch mode PSU's if built corectly acording to the rules are OK, I have some that you cannot hardly hear on my QRM rx, even close up to. In these days of expensive 50hz they represent a considerable saving over the 'dead loss' types.
Insedently the rx I use ( it will only work close up and along cables) is on the WWW " All band rx" Ask Mr Google or Pm me