Exhaust hissing noise

[Guv]

I can hear a hissing noise that I'm assuming is coming from the exhaust when I'm inside the car. I only hear it when pushing the accelerator about half way down (rough guess).

I've checked it from outside the car and can't hear any exhaust blowing noises.

Car is a 1998 Mondeo ST24 (2.5 V6)

Any ideas before I take it to be looked at?


thanks