PSU making a tapping sound?

My build is fairly new and I have a 750W PSU. The PSU is great, a little bit of and over kill for my build I think but anyway, when I do work on my PC its fine but when I start to play games I get this really fast tapping sound which I originally thought was a wire or a zip-tie tapping against a fan and so I took my PSU out (being the Lian-Li PC-Q18 case) to look around inside but couldn't see anything, after putting everything back together again it was still happening and it defiantly is the PSU. I'm slightly worried about this, I'm new to building PC's. Is it something I should be concerned about? Its quite quiet but doesn't sound right to me.

Thanks in advance, Matt.

Yes, unplug it now and send it back