Noise out of nowhere when downsampling

Hey everyone!

I've been using Audition as part of my job for a total of around 11 months. I use it for editing voice recordings and then re-formatting them into various formats. HAd barely any issues with this until today... Every time I've tried to convert into an a-law format (8kHz 8 bit) I get a load of background noise, but only if I close the file I was working on and then open it again. If I listen through straight after converting then it sounds fine. I've never had this problem before , and know to expect a little bit of noise from the downsampling but this is a lot more noticable than normal. I don't get it when converting to similar formats, I've tried 8kHz 8 bit wavs, u-law (8kHz 8 bit) and various sample rate mp3s and all other formats are fine, just a-law with the problem.

Anyone got any suggestions as to what's going on or how to fix it? I've tried adding a gate before downsampling to make sure there's no background noise between sentences before reformatting but that doesn't seem to make any difference. I event tried formatting a completely silent file and it still added noise the whole way through!! Also done the calssic restart computer and close and reopen Audition a few times and that hasn't helped either!

Any help would be massively appreciated!!