79 Bronco with a 400...engine is original and never rebuilt but it's always run pretty well.
Recently started making a racket while accelerating like a vaa-vaa-vaa-vaa-vaa-vaa sound almost like a lawn mower accompanied with a loss of power.
I immediately thought leaky exhaust but I can't find anywhere obvious it's leaking, the pipes are all brand new (1-2 yrs old) from the manifold back so that's unlikely...could it just be a hairline crack in the manifold I'm not seeing? Or a bad gasket leaking? Any other usual suspects I should check? Also, do you have any good suggestions for tracking down exhaust leaks? thanks all

 

Check the valve springs (one is broken)?
Verify with a vacuum gauge to give you another reason to check them