High side low side pressures don not respond to...

Question

I have a new 2stage Ruud heat pump. It runs well in cooling mode but makes a rumbling noise in heat mode. The rummbling noise is in sync with vibrations felt in the suction line. The freon pressures suggest a little low in freon. The addition or removal of several pounds of 410A have no effect on the highside or low side pressures. Current draw of the compressor is 6.5 amps on stage 1 heating and 10 amps on stage two heating. I would not have looked at it if it were not for the noisy operating when heating. What is going on here? Why does it not respond to freon additions with pressure changes or current draw? I have added as much as 6 pounds in this 3 ton unit.