987 fan noise ?

[brucey65]

Hello chaps. I have a 2005 987 boxter S which has just started to make a new noise . With the engine on and air blower fan running I can hear a tinny rattling noise coming from the air vents almost like a fan blade scraping something . If I turn the engine off but leave the ignition on and the blower fan on , the noise is no longer there ?
This has only just started in the hot weather ?
Thanks in advance