The chances are it's a bit of dust or condensation that has crept into the pots on your gear.   Keep in mind that the potentiometers on your POD HD and  the DT amp work digitally.  This means that instead of a bit of dirt in the pot making a scratchy noise (like on analogue gear) a bit of dirt will actually deliver a false resistance value that results in an incorrect digital value for things like volume level.

This is how to fix it...

Periodically, rotate all the pots on your gear a dozen times (or so) through the full range.  This will get rid of anything that might lead to a bad resistance value leading to an incorrect digital value.  If you're using preset controls on the POD HD, rather than setting them as you go, you can park the pots down on zero.  You will notice that the pots have a small null range down near zero and up at 100%, if you park the knobs in the null zones they'll be less succeptable to minute changes in values.  If you do this, it should fix the problem for you. 

If this doesn't fix the issue there may be a problem with your gear.  Give it a go and let us know if it sorts things out for you.

Cheers,

Crusty