Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
I'd ask if the charge could be waived should you buy a new PSU off them, if not, look elsewhere for one.
How old was the card when you sent it back?
You can demand a refund for the card if it was less than 7 working days old (under the distance selling regulations) since you weren't happy with it. Choke noise is not a fault, so they are correct that there is nothing wrong with the card, but if the noise annoys you then you are entitled to a refund (you actually need no reason at all for a refund under the DSR).
PS. do not buy a new PSU, if the card was running fine with the one you have then you'd just be wasting money on something that won't fix the noise anyway.