Mp3 and HDCD

The other night I noticed a distorted sound on some of the higher and lower frequencies on an mp3 I made of the Soft Bulletin (note: these distortions don't appear on the CD). I tried two different encoders (musicmatch and easy cd-da extractor) at all different bit rates (even at 320). It was persistent. Then I noticed it on mp3s I made of the Jack Johnson box-mostly on Miles's trumpet. The connection: they are both HDCDs. So, is this an issue of chopping off the extra 4 bits from the 20-bit HDCD? Has anyone else noticed this? Zach