Shadyacresmack

Hello all. I have a 2001 Ch613 with an E-7 400 and an Eaton 10spd. when at highway speed (only in 10th as far as I can tell) I get an intermittent groan which can last a few seconds to a few minutes, generally under load and seemingly coming from right below me. It will just come and go on its own and I can’t make it stop by feathering the throttle but it just stops sometimes even while still under load. I’ve never noticed it in any other gear including 9th at 50 etc. Any suggestions?