When your heat pump is in defrost mode, does the indoor air circulate in the duct work? Check the copper tubing and see if it runs under the house where the noise seems to come from. If it does, next time you think there will be noise, put your hand on each tube CAREFULLY and SLOWLY. Feel for vibrations and possible pulsing... a burst of bubbles flowing through the tubing. Check the suspension of the tubing.
If the indoor air does continue to flow then usually the furnace will come on. Possibly the hot air from the furnace causes duct expansion (metal ducting) and you hear that noise.