Hey Mrsixstring,

  Are you using the supplied DC-3G power supply for you HD500 or are you using another power source?

  Have you tested all of the guitar cables evolved to make sure it is not a ground loop or poor shielding.

  Please try your POD HD500 in another amps effects loop and let us know your results.

  Thanks.

Many a year ago a guitarist friend purchased a L6 UX Preamp. These are cheap usb interfaces and the noise from the preamp was noticable especially when he upgraded his interface to a Focurite one.

I have had similar issues as yours on S/pdif output noise and my conclusion is that at the price line for the HD's, they are using the same crap preamps which pickup guitar noise to easily. On a clean no amp block or tuner on turned way up the noise is there.

With the Amp block engaged it is magnified the higher the gain model you choose.

The HDPro has a s/pdif in which means a quality DI preamp could be used bypassing the one in the HD presuming it is the same as in the 500. Most likely.

If I were a geek I'd see if a mod could be done on the pre amp replacing op amps for a quieter better one. It is entirely possible if your out of warranty and know what your dong.

In the meantime, if you plug your guitar into the Amp and then FX send to the POD, thru and back to the poweramp section using only your Amps preamp and not the PODs it should be cleaner and prove the prob to be the PODs inbuilt pre amp.

(I haven't tried it as my Fender Amp is noisier than the POD) 

If the noise is there as the HD is connected as above then;

DI boxes come in degrees of quality. Beheringer ones are cheap and noisy and won't make a differece imo.

Go for a much more professional line level isolation transformer in the FX loop send or return. These are simple and you can make one yourself. cinemag etc.

Thought I would add that the loop sends and returns are stereo using Tip, Ring and Sleeve (TRS) connectors.

Using an ordinary guitar jack Tip, Sleeve connection mono's the two feeds and could be a source of noise. I don't know but I presume the HD sums these properly.

Connecting a TRS with mic cable from the HD send and return but only one side connected into your amp TS not joined stereo, only, say left channel, could be worth a try to see if it is quieter. Bare in mind internally with the HD.

Hope it helps! You got a quiet Amp and I understand keeping it that way.