I have a toyota tiger, 4wd, pick up, everytime you put your foot on the throttle at about 1" of depression, it makes this horrible "Chuff" noise - toyota want nothing to do with it, but it sounds like the engine has a manifold leak.

I have checked the intake side and nothing seems amiss.

The only thing I can think of is that it might be down to some problem with either the master cylinder on the braking side when it tries to create a vacuum, as you accelerate, or it maybe due to some sort of breather valve between the crankcase.

It only happens at a particular point in the throttles travel, it cannot be replicated whilst standing still, and it only makes this dam_n sound in 3rd/4th /5th gear - it sounds like something is trying to open a valve somewhere but is just sucking air!

Any ideas?

I am almost beaten, and Toyota don't want to know.

had a similar sound on a diesel engine before turned out to be a burnt out inlet valve

Asuming the valve/tappet clearances are correct, and guessing if its a diesel or petrol truck thats done 12000ks in 20 years, and regulary serviced, there is an easy way to determine where the noise is coming from since we know very little about your truck,

First, remove the air filter, start engine and listen to the intake noise, now, in you ears, are all 4 beats dead even? if so, go to the back of truck and listen to the exhaust note, is that pulsing nicely? or is one pulse a bit lower than the other 3? if all ok, let me know and will go for the next test..

As it only does it when in the higher gears, which means the engine is already running at around 2000rpm for the gear shift and you touch the throttle again you get your noise. Sounds a bit like a little unburnt fuel, from over run, igniting in the exhaust manifold.

Wild guess

Brake vacuum if diesel is provided by a pump.