300tid tappet style noise, but only under load?

A noise that runs at about the frequency of one valve.

But the confusing thing is that it only happens under load. Any load, not just hard load.

So I can be driving up a gentle incline, and the knocking is there, but, as I crest the (gently) hill, and it comes off load (I'm not talking freewheeling down hill, just as it starts to slope down but I've still got my foot in the same place) the ticking / knocking stops.

Put my foot down just gently again, and it's back. I can literally turn it on and off by gently taking it on and off load.


It's quite loud though and I'm sure it's getting worse.


I'm trying to work out what it could be. Things like the vac pump, lift pump, and tappets would not change like this, would they?

The frequency is tied to the engine, rather than road speed, so that would rule out drive train.


Engine starts nicely. I've not spotted any indication of head gasket issues.



Id say tappets but I can't see how they would only make noise under load, and Google doesn't seem to tell me they would either.


Any ideas?

Injector knock possibly.

I fitted recon injectors in 2016, and then the the vehicle was laid up until 2 years ago when I did the bulkhead.

So they've had time, but not miles on them, if that makes sense?


It's so hard to pinpoint a fault that only makes noise when under load. I mean how do you start poking about with your ear in the engine bay when driving down the road!

Stick contact mics on various bits I guess. Not that I'm going to do that.

Thank you.

Check the injector pump timing if it is too advanced that will give you a knock

One thing it could be which is a hard find but costs nothing to check. It can sound like a metallic tap.
Head gasket can fail on piston 4 and compression gasses escape. Find the bottom of the cylinder head and follow round to the rear of the engine. The bulkhead is right up against it so not easy but possible. Put your hand between and feel for little chuffs of air escaping from the engine. Wet hand to feel more if you want.
It's a usual place to fail and doesn't effect the water or oil. Just an age thing. Usually a new gasket sorts it but you can get the head checked for warping while it's off but that failure is usually just the head gasket aging.

Hope you sort it

Head gasket? Blowing out of the rear cylinder by the bulkhead.