High pitched whistle

Today l was out in the Defender TD5

I got to a hill and was flat out in fifth at about 45 mph. Normally l would have changed down, but l left it as l thought it would not do any harm.

Anyway, after about a minute, the turbo began to make a loud high pitched steady whistling noise.
Well, l assume it was the turbo. I got to the top of the hill and backed off, the whistle persisted for a few seconds then just suddenly disappeared.

I have checked the manifold and there is no visible sign of gases escaping, or loose bolts. Could it have been the wastegate sticking? It did seem down on power, l would have expected it to accelerate on that hill in fifth gear.

Is there anything l could check? The Defender is still under warranty but the noise has not occurred since.

Maybe l should go back and try the same hill again?

Sounds like a loose turbo hose letting the pressure leak out and whistling at the same time.
Also flat out in fifth at 45mph does not seem right - you should be doing well over 70.

Well, I go up hills in my TD5 in 5th and keep my foot down and the speed decreases gently even though the accelerator is fully depressed. So I think that's what Lightning means. Once it gets to about 40 I change down. Mine doesn't whistle though!

I just drove the hill again, in fifth at 45mph as a test, and this time there was no whistle.
Also the speed crept up from 45 to 55mph

I guess l will just see how it goes, but l will also check the hoses as suggested.