Well I replaced a 2000 2.5 SOHC in my Forester with the same engine except it's a 2004. Everything went well, it runs fine but there is a noise I don't remember hearing before. It is kind of a low humming sound, it is NOT rpm or even engine related because I can get the sound briefly with the engine off. After turning the key to on position it happens for a few seconds then stops. While the engine is running it is continuous, same pitch, same volume.

 

Does the fuel pump sound like a possibility?

I can't see how a swap would cause any change in the fuel delivery system, maybe it was always there and I'm listening more closely now... Any ideas? Thanks

Some IAC's are noisy - was it noisy before?

 

Any CEL's

 

Just guessing/asking.

Check the ABS pump. Use a screwdriver as a stethascope and see if you can find the hum.

 

 

 

nipper

Air box sealed tightly? We have some broken clips on a '98 and it hums where it doesn't seal tightly around the filter...well, before the screws it did.

Sounds like maybe ABS or fuel pump. Did you try to pull the ABS fuse(s)? I would imagine that you could pull the fuel pump fuse as well while the car is running, it should run for at least a second or two... maybe...

If it is an automatic, you could be hearing one of the duty solenoids. They hum pretty loud on some cars.