1999 Volvo S80

There is a hissing noise when I press the gas pedal. It gets quieter
when I don’t, louder when I press harder. It also gets louder the
longer I drive. Sounds like a soft hiss, and then turns out into
something like scratching of sand on glass.

Anyone has an idea what it could be? It’s not there when the engine is
cold, so the service guys didn’t hear it (they didn’t test-drive it
long enough).

I’d really appreciate the help.

Last post for today. Merry Christmas everyone and goodnight.