Rumble in engine when accelerating?

[SirPudwhip]

I have a 2000 4.0L V6 4X4 OHV. When I am going under 50 or 60 mph, the vehicle runs fairly well, however when I try to accelerate at higher speeds, there seems to be a rattle/rumble coming from under the hood. I suspected that this was the motor mounts, but I looked at them this morning, and the rubber pieces on them did not seem torn. When I hold down the break, I can put the gas all the way to the floor without any rumble. I do not know what is causing this, and was wondering if anyone knew what it could be

 

[SoNic67]

Exhaust leak (manifold gasket) or lowered oil pressure (lifters chatter) will do that. On V6, the timing chain tensioners might suffer with low oil pressure.

 

[SirPudwhip]

Thanks SoNic67, Ill look into that

 

[JimMadsen]

Is there a sticky or how to on the manifold gasket replacement? I need to get around to it eventually myself

 

[SoNic67]

I paid a mechanic in my case just because the manifold bolts usually are so rusted that they require a torch to come out...
My bolts where a mass of rust.