Help! (buzzing and crackling while recording)

I'm not very technical and there's probably something blatantly obvious about what's wrong. I know just about the basics of this system so forgive me if I come across as a muppet.

When I try to record an audio track I get horrific buzzing and crackling through my monitors/headphones. All works fine until the red light goes on and then it starts. Audio instruments work absolutely fine and midi devices do too until the moment I try to put something to an audio track. If I play a guitar, it's clear as a bell but when I go to record I get the buzzing from hell! If I open a new song, the buzzing happens and on certain other songs, where once I had no problem, it buzzes on audio. Certain other 'old' saves, it doesn't buzz on audio at all!!

Where the hell do I start with this?? I've checked/changed if necessary all the wiring and it's not that. Any help at all would be greatly received, it really is doing my head in.

Thank you in advance.

I have a very basic set up. It was set up for me by a studio engineer a couple of years ago. I since moved house and dismantled it then put it back together. It did work fine until the past couple of weeks when the buzzing started.

I have a M-Audio MobilePre USB plugged into a G5. I have a keyboard, guitar, mic etc going into the MobilePre and Alesis monitors from the output. Like I said, any devices are clear as a bell until I hit the record button and it buzzes crackles like mad. I was thinking it might be the MobilePre that's gone but surely I wouldn't be able to put anything through it if that was the case? Does the MobilePre need power from a source other than the USB lead from the G5? I've tried a few things within Logic but to no avail.

Not sure what you mean by software set up? I'm sorry if I sound vague, after all it's me who needs help. I'm grateful for your reply.

Just for the record, I'm working on a song and the audio is fine. I have another song started the same day and it buzzes like mad. I opened a song from two years ago and it buzzes like mad. I opened another song from two years ago and it's fine

In answer to your questions, yes on both counts.

Last time I looked you asked two questions!

The power's fine, the M-Audio only has two jack inputs and 2 mic inputs I've rotated them and I've had brand new leads. Like I said, I'm not very technical but my gut feeling is that it's something within Logic.

'what is the sample rate and bit depth you're recording at? is it the same for all songs, or do you change it per song?'

I really don't know about sample rates or bit depths. I haven't conciouscly changed anything, is there a default sample rate and bit depth?

How do I get in to start fiddling around with coreaudio or is that not recommended?

I bought a thing called a SaffireLE on recommendation, whatever that is but it's beyond plug and play unfortunately. My Mac recognises it, asks me if I'd like to use it instead of the MobilePre, tells me something about inputs/outputs having changed and then I can't get any sound at all when I try to use it because there's obviously something else I need to do that I don't know about! The instructions are non existent other than 'plug it in and your Mac will identify it'.

If I got that thing working and the buzzing was still there, then at least I'd be 99% sure it's not the hardware that's faulty.

'can you post an mp3 of a section that has the problem? sample rate mismatches have a characteristic sound and we might be able to cross that one off the list right away.'

I'd love to but I might have to wait until my son rises in the morning to show me how the hell to do that!

I'll have to root out the M-Audio CD and the Logic one too, I'm not best organised. I got the upgrade from Logic 6 to 7 about six months ago. When it was emagic, there used to be a support number you could call and they were unbelievably sympathetic and helpful AND it wasn't even a premium rate number.

On Tuesday, I went into the Apple Store, Manchester. Unfortunately, the Logic guy was out all day so I couldn't run it by him. They offered me a support number but it would cost me £149 ($300) just to call it! Sod that for a game of soldiers!

Well before. The only thing outside the computer it could possibly be is the M-Audio, that's why I got a new one. The guy in the shop mumbled something about the Saffire being 32 bit as opposed to M-Audio being 16 bit or something like that, so it'll be better anyway.

I did call the guy who set it all up for me and like yourself, because you can't see or feel it, it was impossible for him to establish what could be the problem other than faulty leads etc. Problem is, he's 100 miles away, about a third the length of England!

I was kinda living in the vain hope it might be something someone else has suffered and could be fixed at the flick of a switch. Inevitably, it'll probably end like that!

I have Logic 6.4.1 on my computer and I just opened it and recorded to audio with no problems at all via exactly the same set up as previously described. Crazy. If everything in Logic 7 buzzed and crackled, I'd feel better in a funny sort of way but because some songs do and some songs don't, I will continue to lose sleep