Heat pump is doing this when the unit shuts off in heat. Does not do this in cool when the comp shuts off. Reversing valve energized in cool.

Customer had a dirty ass filter and that dropped the high side pressure from 350 to 250 (during heat mode) on filter change... i noticed that the unit seemed a little low on refrig since 7* SC in 50* ambient (in cool mode). Added 2lbs of R-22, got the SC to 10* (in cool mode).

Anyway, with the pressure drop the "water hammer" is less prominent. Just started doing that this season. Also found a leak at the TXV distribution tube fitting.

Does low charge cause this due to lack of column of liquid?