2004 Grand Marquis high pitched sound near firewall.
 

Hello.

As soon as I start my car and constantly while it is running, I hear a very high pitched sound that seems to be coming from near the firewall by the transmission fluid dipstick. The sound does not increase when I rev the engine and varies slightly while driving. Makes the same sound no matter if the A/C or heater is on or off. I have a stock 4.6L engine. It also triggered a check engine light.

Anyone have any thoughts as to what it could be?

EDIT: Problem fixed. It was a hose going into my passenger valve cover.