Hissing when off throttle

[Robh18]

Is this normal ?? When driving (more noticeable in 5th gear going down motorway) if i come off the throttle the T makes a quiet hissing type noise from I think is coming from engine bay. If I then go back onto the throttle noise disappears !!
What is it ??

 

[Cue]

It could be the whistle from the passenger wing mirror?

 

[ste1985]

Most likely as Cue said, mine some times does the same time to time and never found anything to say other wise.

 

[eddyg]

Double check all your vacuum hoses etc into your inlet, although that would result in other issues!