Landroger,
The air for the air conditioning, pressurization, anti-ice etc is supplied from the compressor sections of the engine. On the type of engine that I am familiar with, the air during pressure normal power settings comes from the IP (intermediate pressure) section. However, during low power operations such as descent, there is not enough bleed air available so it is augmented with air from the HP (high pressure) section.
During operations in the terminal area where you may be carrying out step climbs or descents, as the thrust is varied, the HP bleed air valves will be opening and closing to make sure that sufficient air is available.
I think it is this that is making the strange noises.
Rgds,
Dean