Engine knock

just a little curious here, ive got a inconsistant engine knock/rattle that someone (tweake lol) mighyt be able to diagnose.
i comes about when towing mainly, at 58km/hr in third it will start to come in and about 78km/hr in fourth. if you back off the throttle a bit and let it accelerate real gently in both gears, you can actually get past the point (64km/hr and 74km/hr) where it rattles and will continue to rev hard without a sound. if you continue to push it as it starts to rattle/knock gets real hard and nasty. the motor isnt labouring at this point and you 'control' the knocking with throttle percentage i;e u back off it goes quite, accel harder and it gets worse. change back from 5th to 4th at 95km/hr she will rev along happy but as soon as it gets down to that 74km mark in comes the rattle. but only get it faintly once every blue moon when not towing. thought it may have been timing prob, but doesnt seem like it. from working with diesels, its sounds for the life of me a diesel thats running lean, dont think its overfueling as returns great L/100 and doesnt smoke even when its knocking.... any ideas?

what RPM is "58km/hr in third" and "78km/hr in fourth" ?

sounds like to much EGR. could be blocked up air intake ie dirty air filter.

prob would of helped in adding the rpm lol, round the 2750-2900 for both gears. she gets a new air filter every 15,000 and this one is less than 6000km old, still clean as a whistle, first thing i checked and replaced when first noticed the knock about a year ago. im going to take it around to a diesel specialist just for a health check when i get time, might fit a catch can to eliminate poss of it sucking oil thro and combusting it.

i would block off the EGR just for a quick test. the only thing i can think of is egr butterfly opens @ 3000rpm. i wonder if the exhaust is a bit blocked or EGR valve a bit sticky and its getting to much EGR. 3000 rpm butterfly opens giving it more fresh air.
with light throttle you probably don't get the high exhaust backpressure which shoves the EGR in.

also check the exhaust make sure you havn't dented a pipe/muffler.

so that explains the power increase and easier reving that you get at 3000rpm, its actually getting some fresh air!. yeah ive gotta drop the oil this arvo so ill have a bit of a poke around and see if there is anything obvious, was just interested in getting ur ideas on it, not really concerned at this stage cos mainly only comes about when towing. thanks tho.

so that explains the power increase and easier reving that you get at 3000rpm, its actually getting some fresh air!

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yeah one way of looking at it. tho its more to do with fuel tuning. it still has that 3000 rpm power band even with egr removed.