My 2006 X Type 2.2d Estate has developed a "ticking" noise in the engine. It sounds like a valve lifter or possibly an injector, but I have been all over the thing with a stethoscope and can't find an obvious source. Any thoughts?

 

Timing chain guides

 

I usually use 2 types of stethoscope:
mechanics (with a stick)
"medical" (no stick). I stole toy stethoscope from my daughter, it works fine.
Another way to locate sound is to use camera. Move camera around engine and, when you watch video on computer you'll hear where the sound come from. Post video on youtube and link here.

 

Thank you for this suggestion, but I suspect it's something else. The ticking is regular and at half engine speed which suggests a possible faulty valve lifter. My stethoscope is the mechanics' type with a probe and I've been all over the engine and ancillaries without being able to locate the source.

 

Lifter noise is usually easy to locate, including cylinder, there should be big difference between heads.
Crank knock is harder, did you check oil pan?