High pitched hissing/check engine light

[neuroru]

Hello all,
I have a 2001 Echo, and a few months ago it started emitting a high pitched sqeaking/hissing sound. I checked the fan belt, but it turns out the noise was coming from the engine. My brother-in-law told me it could be air escaping from the intake valve and it would be fine as long as I could tolerate the noise. But yesterday the check engine light came on, so I'm worried his assessment may be incorrect. I can't afford to take it in for diagnostics right now. Does anyone have any ideas?
Thanks,
Neuroru