From the fan which sucks air into the heat pump, to the compressor which creates additional heat to warm your water, older heat pumps can be quite noisy. This is especially true if it has been installed near a bedroom where you need quiet.
Modern day heat pump hot water systems, however, are only slightly louder than a whisper. Despite this, poorly placed older heat pumps have given this hot water system type a bad rap, and many people shy away from them because of this.
If noise is still a worry, consider having your heat pump installed away from living areas such as bedrooms and living rooms to avoid noise being a concern all together.