Engine noise

This van I bought has a high pitched whine or whistle when driving
around town. It occurs when you step on the gas. The faster the engine
turns the more high pitch it becomes. When the foot comes off the gas
it becomes louder but lower pitched for a second or two. At idle the
noise isn't noticed and isn't heard at highway speeds. I'd like to know
whether it's a noisy alternator or a vacuum leak? I was assuming it was
the alternator but then it could be associated with the check engine
light that came on once recently, showing code 0121.