Continual whistling, about 1 peep per second????

[MortCymro]

Hi
Started the bike recently and heard a faint but persistent whistling: "peep"...."peep", not related to revs or road speed. Bike performs perfectly otherwise, and I can only hear the noise at low revs, but what the #$!@ is it?
Biking son suggested air emerging from the tank - I'd filled up recently - but opening the lid to release any pressure made no difference.
The bike had tipped over against nearby shelves at home when I got off it after filling up that time, to about 45 degrees (sod of a job to pick it up, too, after squeezing in between the bike and the shelves) but I can't think how that would be relevant.
Any suggestions gratefully received!

[Art]

The way you describe it it will be air or gasses escaping from somewhere. If I was a gambling man I'd wager the exhaust was disturbed when you dropped it and what your hearing is an exhaust gas blow by. I'd put it up on the centre stand and run the engine to get a fix on where the whistling is coming from and take it from there. Once you're in the right area diagnosis should be a piece of cake.

[jm2]

Has your bike done many miles ?

I do wonder if your pump is complaining/on the way out.  They sometimes sound noisy for a while (sometimes forever) before they go low pressure.  I'd also prop the tank up and feel around the fuel connector checking for any dampness - just to make sure it isn't weeping.

[MortCymro]

Thanks, Both
Started up today to check exhaust system as Art suggested, and noise had gone! - Mixed blessing, as sporadic faults are the worst, as I hardly need say. There's no smell of petrol around the bike, but I'll check carefully - there was some spillage when I tipped the bike, which I assumed was because it was filled up 2 miles before. To answer jm2, it's done nearly 26K miles.
Thanks again: no doubt, like Arnie, I'll be back.