Toyota Hilux Tiger - 2002 - 3.0 EFI Diesel, 4 x 4.

What a great forum, I am new here so if I have posted this in the wrong forum, apologies, please move it to the appropriate place.

I have an annoying problem with my Toyota Hilux 3.0 EFI Diesel. The problem is only detectable when running with the engine under some load, and usually appears in 3rd/4th and 5th gear.

As I accelerate, somewhere around 1200-1500 RPM, there is an audible "Click" which seems to be dependent on throttle position, i.e, it always occurs at the same point of travel on the throttle position (could this be a throttle position sensor?)

At this point, there is an audible "Chuff" noise - like a vacuum operated device. The engine sounds rough, but is not missing or spluttering, it sounds almost like an air leak on the inlet side.

The "Chuff" noise only last for a second or so, and cannot be reproduced in neutral, first or 2nd gear.

It sounds like there is some sort of valve or similar - like advance or retard or breather valve opening at this throttle position. The noise has only recently started and is quite annoying.

I have no experience working with fuel injection, but would still like to be able to give it a go. If anyone has any idea as to what this problem could be related to I would greatly appreciate the help.

The engine only "sounds" rough, there is no excess smoke, no hesitation nothing to indicate a problem other than the click "chuff" and roughness. I just don't want to incur any more damage if possible, and would like to get it running nice and quiet again. (it has 200,000km).

Many thanks!