Please help with 'burned in badness'

I don't hear it on monitors, I only hear it on headphones, and the volume has to be turned up a little bit. It sounds like a compressor or a limiter kicking in, but that isn't really possible because my compressor is extremely gentle and I'm not using a limiter. It is subtle to be sure, but I can hear it. It's like a shallow muffling sound. Around 3 or 4 seconds in.

Anyone? Out? There? Someone must have an idea...

Thanks schwa, Are you sure you can't make a 'Fix unknown frequency problem caused by some outside force' plugin? Since the effect almost sounds like compression to me, would an expander help? Perhaps SStillwell expander or autoexpand?

I agree with the sock puppet. :) If you can clearly identify the sections you can chop up the digital copy, put it on different tracks and try to match it up with EQ.