There's nothing wrong with the standard Nissan timing belt - dead quiet at idle and has a 100,000k change interval, I got mine for $85.00 off eBay, Nissan sell them for around the same.
That being said, my R33 had a very annoying whirring at low revs when I first bought it, and it apparently had a new, genuine Nissan timing belt.
I eventually took it to Nissan and had them replace all the belts and tensioner/idler pulleys as part of a major service...It turned that the tensioner bolt was slightly bent which was causing most of the 'hum' or whirring at idle, it's perfect now - might want to check this out if you're having this problem.