Are you sure this problem occurs in the recording and not in the replay? It's the typical symptoms of poor regulation in a power supply, usually in a part of the chain that takes extra current when the sound is loud, such as a power amp or headphone drive circuit. It could yet be in the Tapco interface during recording.

Try burning some sample tracks to CD at various levels directly from your computer, then take the CD to several different replay systems (friend's hi-fi etc) and listen to the result.

^
I've just put the .wav file on a USB stick and listened to it on my regular desktop PC -
But the problem still occurs. :/

Im trying to upload a sample but my connection drops out every time. Its only 1.29mb :/

...not when the notes are played very softly but when the notes are struck hard it appears all of a sudden.

What size is the room you are in? How good is your technique when playing soft or playing loud? Are your mic and the computer in the same room? Do you use headphones and/or monitors?

Thanks, I'll try and answer those.

Roomsize is about 5 by 9.

There's nothing wrong with my technique as far as I can tell. I have taken lessons and everything :p
It's a nylon string acoustic in this case, and using either pick of fingers. It appears with singing as well. Just repeating the same word very softly and then loduer and louder there's a certain threshold in volume above which it'll add this whitenoise-like hum but it's just for a sec because inbetween words or loud sounds it disappears.

And yea, themic and computer in the same room, but I turn the screens off when recording, and I know the the hum effect electronic devices produce when recording, and this is definitely not it. It would not sense to appear on high input only, anyway.

I'm using headphones on the laptop and a Hi-Fi system on my regular desktop PC which I use for doublechecking on issues such as these (mastering/EQing etc is mostly done at a friend's).
And I'm kindof lost as to what to do :/

bornfidelity wrote: I've just put the .wav file on a USB stick and listened to it on my regular desktop PC - but the problem still occurs.

Sounds to me as though the problem is in the Tapco box, probably associated with the power regulators. Can you borrow another different type of interface to try in place of the Tapco (keep everything else the same) and see if you still get the hum?

By the way, are we talking genuine 50Hz hum, or just some ill-defined fuzziness during loud passages?

It''s close to 'white noise'. But I've contacted TAPCO support by email, and I'm hoping for them to respond in the next couple of days :)

"White noise" and "hum" are worlds apart. It's less likely to be a power supply problem if you are now saying the effect is like an increase in background noise level when you play loudly.