Do you think the installing the base plus the additional rubber pads - made a difference the resonance?
The base made about a 10% difference, but the legs on mine are hard plastic. The EVA pads made 70-80% difference and finally allowed me to run it at night, I was out of heating season then. I was able to shift the resonance aswell with the flashing thing, which also helped alot.
The rpm fix happened at some point after, so looking forward to trying it this winter.
My into has no pipe coupling to the wall at all, as it’s through and through.
If I get some time I will also buy and install the calculated springs, but that’s abit more involved since they are higher than the travel of the feet. Whereas the pads were easy to wind up a leg at a time and place under and wind down.
I am still surprised yours is the compressor. It might make sense to tie your pipework past the flexi to something solid. Which will work as long as the plant side is isolated.
You should be able to shift the resonance itself by placing something heavy ontop of the unit, this would be like a mass dampener(?) Someone will correct me on the correct terminology and why that works. Unfortunately the top of a cosy is curved so not as easy as with a box unit.
I say calling it a “damper” is somewhat misleading. It does not damp the oscillation in the sense of lowering the Q and widening the bandwidth of the peak, but moves its natural frequency away from the frequency of the source of the vibration, hopefully reducing the amplitude.
I guess you could demonstrate with a handful of rubber bands and a weight: suspend the weight by a few rubber bands knotted together and bounce it up and down. Change the weight and (ignoring the static stretch) bounce it up and down again and the frequency of oscillation will be different. If you keep the same weight and change the rubber bands (either add more in series or two strings in parallel, again the frequency changes. This is like changing the feet on your HP.
You could even translate distance, the mass m and the k of the spring into electrical parameters i, L, C and simulate it in LTSpice 