Sound? Problems?

Hey,

I just have two questions I was wondering if anyone could help answer.  First, I hadn't really noticed until someone pointed out that my PC (dimension 4600), while it's running, makes this really high-pitched, continuous ring (i think it sounds something like a metallic object vibrating) and the sound sort of comes in subtle waves back in forth. Is this normal?  If not, how could i fix it?  It seems like the computer has made the sound since I first got it (a little over a month ago) but now it appears to be more significant or maybe that's just me being more aware.  Second,  I've experienced something that hasn't occurred all the time but just in the past week or so.  When my computer starts up, at the Windows XP screen, the screen flashes and there proceeds to be a little pop in the speakers.  Afterwards, everything runs as usual but, every once in awhile, the speakers will continue to pop or crackle.  If anyone has any advice on or answers to these inqueries please let me know.  Your help is much appreciated!

Thanks,

Rio424