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As it is a USB mic the audio doesn't go anywhere near the laptop's on board sound card but directly from the mic digitally via the USB into your recording software. It sounds more like acoustic background noise like a fridge or boiler or something else in the room. Problem is once you try compressing it it may sound even worse since you may hear it pumping with the compressor. Best to try and remove the noise at source when and where you are recording.
Thanks very much for your reply (and sorry it took me an age to get back to you).
It's really helpful for me to know that it's not soundcard stuff, but ambient acoustic stuff.
I'll try again and make sure I turn everything off in the room.
Besides turning things off, a few other things to think about:
First, sound follows the "inverse square law" which means that every time you double the distance from a sound source, you quarter the amount of sound you get. If, for example, a laptop fan is an issue, getting farther from the laptop (and yourself closer to the mic) can really help.
Second, most USB mics have a cardioid pick up pattern which means the direction directly behind the mic capsule has very little pick up. Arranging your recording session so the mic points directly at the sound source while the back points at the worst source of unwanted noise can also help a lot.
Third, most domestic rooms are acoustic nightmares. The real solution is to properly treat the acoustics but this is rarely possible in an average house. However, you can help the acoustics by putting soft material behind a person speaking and possibly to either side but not behind the mic. An easy way to do this is to use cheap PVC pipe to build a couple of L shaped frames and hand duvets or movers blankets from these. If you don't glue the 90 degree corners of the frames, they can be dismantled and stored under a bed or whatever.