There's no such thing as a silent PSU at a reasonable price. That's unless one's fortunate enough to pick up a secondhand bargain on eBay or similar. E.g. £90 for totally silent, fanless 300-500W ones at click here & click here.
Scan are doing a few 'Super silent' Silverstone PSUs on their Todayonly page click here, starting at £50 that could be amongst the quietest non-fanless PSUs. The 350W Be Quiet one at eBuyer for £30 click here might be reasonably quiet too. Also consider this £40 400W Corsair click here. All the above make quality PSUs & one can't really go wrong with them for the money.
If you want zero noise, you'll need to stump up the cash for one, or be patient for a bargain in the auctions. G