1997 GMC Sierra fuel pump diagnosis

While engine is running, at idol, in park, there is a whining noise coming from underside of truck. The noise is gradually getting louder. There is also an interferance on AM radio. I have been told, by an amature mechanic, the noise is the fuel pump. No noise is noticed when key is "on" but engine is not running. No problem with accelleration, idoling or cruising. Fuel pressure has not been checked, how much fuel pressure should I expect?