I bought an Audio-Technica ATR-6550 Condenser Shotgun Microphone off eBay which works fine most of the time but sometimes picks up noise as heard in this recording. Can anyone identify the problem?
This sounds like a bad mic capsule to me. However, it could also be caused by a bad phantom power source or bad preamp, and so you'll need to do some troubleshooting to figure out which one. Can you borrow or purchase (with a return policy) another mic to try? Are you plugging it into a video camera (the mic's intended use) or something else?
The noise you can hear is someone snoring. My advice would be to attend your local GP for assessment and possible treatment.
There's also a bit of static on the track, which is possibly due to a faulty capsule or excess humidity.