Bongo Fury

On the way to visiting my daughter, just 9 miles up the road, I decided to give the old girl (mazzie) a blow-out. Reached the dizzy heights of 70mph and going along nice and smooth. Just mentioned to my wife, Brenda, that I believe that Mazzie seems to be at her happiest at high speed; all of a sudden a hissing type sound came up as though air was leaking from somewhere at the front. When I lifted my foot off the accellerator slightly the hissing sound disappeared. The radiator wasn't overheating; there should be no leaks anywhere as I recently had to have a new inlet manifold gasket and new head gasket and everything should be tightened up. The same thing happened again on the return home, this time I stayed at 70mph and within 30-60 seconds the hissing at gone! It is still a worry though when something like this happens. Any thoughts or comments on this ghostly happening would be welcome.

Snakes on a bongo?

The turbo is under the passenger seat, I think I can hear the "hiss" of it spinning up and down if I play with the accelerator while keeping it in top gear...?

Thank you, Driver + Passengers, for answering my Techie query. Not knowing anything about turbos and their workings you have made me happy. Don't much like strange noises. Now for that scrapping sound.......