bigtow

I have bought a 2004, 2.5 manual outback. Great car everything fine for 80k miles, good economy, but, whenever I accelerate I hear a well defined purr from the engine that gets faster as I accelerate. No noise when I press the accelerator in neutral just when moving. It isn't deafening just intrusive and I can't hear it when cruising.
Also the engine warning light has come on with flashing cruise control light.the the sensors were (reconfigured)? , twice in the past 4 weeks. A mechanic thought it was the exhaust manifold gasket and replaced it that was causing the noise and the warning lights, it made no difference. Have you any thoughts. Sorry such a long first post. Thanks Bigtow