Hard Disk Monitoring

1.5TB WD Green drive. Did a full READ+WRITE+READ scan two days ago. The procedure completed with zero errors, no slow areas, no SMART changes; everything seemed perfect.

After this test however, the drive keeps making noises similar to when it's being used. They're not clicking sounds, and they're not those read retry sound like when you have a bad or weak sector. It's simply normal access sounds.

The only times when the noise stops is when the drive is actually being used. Then, after a few seconds, it starts again and never stops... it's like the drive saying "Okay guys, I'm done here, let me get back doing my own business"... whatever that may be.

THIS IS NOT OS-RELATED! The drive does the access noise even with the SATA cable disconnected. Again, no errors, but the noise is annoying.

There's still about a month left for warranty, but I wouldn't want to go through the hassle of RMA-ing it if not necessary. Any idea what's going on with the drive, and when/if the noise will stop?

Did you start Disk menu -> Short self test / Extended self test functions and sent it to background?

The continuous noise may be related to a background hard disk test which runs "inside" the hard disk, just like the above mentioned Short self test / Extended self test. These are running when the hard disk is idle, when there is no other hard disk activity (no reads/writes).
And upon reads/writes the test may be paused and resumed automatically, so they may run for very-very long time (even hours or days).
Usually complete power cycle (complete power OFF and ON) may stop this (simple restart / reset does not) but depending on the hard disk actual model, firmware, the test may continue running until it's completed.

You can verify easily on the Information page, just scroll down the "S.M.A.R.T. details" section.
Personally I'd recommend to use Report menu -> Send test report to developer option, then it is possible the actual status and verify if this is the case: if the hard disk reports that a such background test is active and check the actual complete status of the hard disk itself.

If the test seems not running, then I'd try to perform a complete power cycle (just to be sure) and then use Disk menu -> Short self test, to initiate a such (but short) internal hard disk test which should really complete in some minutes - and then hopefully there will be no continuous background activity causing that noise,