85 4ksQ(uestions)??

[only5kgt@uswest.net]

Hello again!
Today, when stumbling around a pick-your-part looking for a
spare afterrrun temp sensor for the 5kcst (thanks to Wallace
White for the hint BTW) I was greeted by the eager but
seemingly clueless (I could go into this, but I wont. WOB)
yard owner: "So, you like Audis? Take a look at this..."
Lago Blue shiny 1985 4ksq!
Hmmmm. This was a nice one indeed. $1800. 191k, but in
better cosmetic shape than my 5kcst by a good margin.
Started, stopped, all quite well. Cold AC and every single
accesory worked. I began to get my gears turning on this
one.
A few questions for the 4k conversant (all my Audi
experience has been type44)
Persistent tapping from top end. A little louder than the
manifold noise on my 5kcst. I listened with a screwdriver
against the valve cover, head, intake manifold, and vac
pump. All quiet but for a little ticking (thru the
screwdriver) on the pump housing. Think it's the pump? Motor
sounds fine otherwise, no smoke, good power.
I removed the oil cap with a running engine, and it kept
running. On my 5k, you can't do this without stalling.
Normal, or is something amiss in the vacuum system?
Fuel pump is making that familiar old Audi buzz. Is R&R
harder/easier/the same procedure as on a 5k?
Finally, and I think most problematic: I don't get anything
happening when I move the diff lock switch. No lights,
nothing. Didn't behave like anything was locked on
slow-speed maneuvering, so I don't think I'm hurting it this
way- but how hard is this setup to diagnose (85 1-2 switch)?

Sorry for the relatively silly questions. Aside from these
above items, the thing is so solid and clean, it's amazing.
Also, what is the collective wisdom on the value of one of
these beasts. Comparatively, what does a 5kcst with similar
miles fetch considering it's got issues with both hydraulics
and AC?
I've already thought of making a 4ktq of these two cars, but
aside from expense, I've got nowhere to keep such a project
right now!