sounds like a classic case of gain problems. Pls remember that most all recording have a little hiss in them, it may be that these tweets are so much better than anything you might have had in the past that previous speakers just never picked up the hiss in the recordings.
Get a non-copied version of dire straights brothers in arms CD, iirc there is no hiss in that recording. That might narrow it down to whether it's poor recordngs or sytem gain problems.