Thinking back awhile, my manifold distorted after 6 years or so and blew past the gasket and it started with a whistling sound under acceleration and slowly got louder and worse over a few weeks. The cast iron manifolds are prone to distorting due to heat induced stress relief and in my case it snapped off one of the studs into the head.

If the manifold has distorted don't bother getting a new one as it will inevitably happen again just have the old manifold flange face skimmed in a milling machine as it shouldn't ever distort again plus the new Rover part costs a fortune.

 

So I've finally found the source and as you see in the picture it's coming from what seems to be the bottom of the engine below a spark plug. I've tested this multiple times with putting my hand down there, it's not coming from the manifold gasket or the manifold itself.

 

Uggh, sounds like the HG unfortunately where it's located and I was hoping for you it wasn't. Strangely there didn't appear to be any misfire but it was only on tick over. Let us know where you finally pinpoint the noise.

 

time to take the heads off

 

Yep. I have heard that noise before.
You must stop driving it.
head must come off.
Put in Victor Reinz Head Gaskets.
Not unknown brands made in China.

 

You must mean OEM LR gaskets then