I own a 2004 Mazda 3 that I bought used with 164,000 miles on it. After about 2k miles, I began to hear an odd noise when Idling. The noise is hard to describe, but it goes away when I push in the clutch, and it slows down when I push in the clutch, then comes to the stop. Almost like something is spinning and slows to a stop. The noise has become louder and I'm at 171,000- miles.

Also, about 500 miles ago, my car would begin grinding going into fifth gear if I'm above around 45-50 mph.

 

Sounds like the release bearing for the clutch.

 

Agree, sounds like you need a clutch