It kind alooks like a DC offset. It's too hard to say just seeing that portion. If it is consistant throughout you can try running a DC offset on just the left channel and see if it is noticably better. I think it's possable to have the "rarefaction" of a waveform differ from the "compression" which would look like that.
All DC offsets I have come across personally were from analog sourced material that had been transfered at some point.
as bean asked, do you hear a problem?