HVAC Return Noise

Hello, I've been trying to troubleshoot some noise that's being generated by my HVAC system in the studio. I've found the source to be the return in my live room. The return is about 12' of 14" acoustic flexi duct. The duct penetrates the machine room about 5' from the air handler. The noise I'm fighting is a low freq. rumbling that's being transferred through the return, even with the grill off. I'm pretty sure what I'm hearing is mostly air noise and not the airhandler... I could be wrong. In any case, I'm wondering if there's a simple modification that I could make that would help remedy this noise. For example, add more length and turns to the acoustic flex. Or change the duct work to square acoutic lined duct. Or will these things just add to the problem or just change the sound and not get rid of it... I've read about a folded horm plenum that Rod designed in the forum. I think I've got room to build something like this, ceilings are 8' and I've got 5' between the air handler and the wall where the return penetrates the live room. I'd prefer an easier solution:) Rod? thanks:) andrew