Engine's hum is high pitched

I'm looking for a new car and recently test drove a 2004 mustang. I liked the car a lot--it had low miles and the dealer said he was willing to deal with me on the price--but I couldn't get other this kind of high pitched hum that the motor had.

It wasn't like a really high pitched, shrill sound or a whistle, but it was very noticably higher pitched than any engine I've ever heard. My dad, who knows much more about cars and engines than I do, didn't know why it sounded like that either. It wasn't something you had to strain to hear, it was very noticable.

Does anyone else have a mustang or any other car that sounds like that? It's hard to believe that the cars engine just naturally sounds like that, especially on a respected sports car like a mustang. Does anyone have any ideas on what could be causing the problem?

Thanks in advance.

Was it supercharged or did it have an aftermarket intake system/air filter?

Was it supercharged or did it have an aftermarket intake system/air filter?

I really don't know. If it had a supercharger, the dealership didn't know about it.

I was also just thinking, and I think that I mainly just heard the sound when it was idling and when I was accelerating.