power problems

[domrover]

I have a 75 2.5 v6 petrol on a 52 plate I am losing power at the top end when the engine is warm and it souhds like a desel and the engine makes a kind of hissing and sucking sound when i rev the engine but the engine does not rattle that much at all. I have been told it could be the inlet manifold as the butterflies can break up can any one help please
Thanks

[trikey]

It could be inlet, but that would not cause any loss of top end power.

[oldcarguy]

Sucking and hissing points to induction, check throttle body/pipes/inlet manifold.