Land Rover Repair: 98 disco 1 tapping, loose bolt, side fender

Question
I have a disco 1 that taps at warm up. it is hard to pin point, it is loudest at passenger side fender well. the exhaust does have a leak on this side but it goes away at warm up. i have listened to all the lifters they sound good. it is when i stick a screwdriver between the starter and the block from underneath, i hear the noise there inside the block. i did a recent engine flush, and some think the oil pump pickup may be clogged, some think the cat is making noise. when you listen to the exhaust, all i hear is exhaust sound, no extraneous noise. no lights are on on the dash. i am going to remove the oil pan, i have no idea what i will be looking for, i figure there is a screen for the oil pickup that i can scrape gunk off of? what else do i look for, and do you have any ideas how to better diagnose? 128000 miles 4.0 engine. thank you in advance!

Answer
Hi John,

Could it be exhaust crack/leak, rockerarm or loose bolt(s) on a pulley?  Did you know that sticking exhaust valves are a common problem with Disco's?  

Worst tapping noise I would ever suggest is a loose cylinder liner - that spells new engine or complete rebuild.  

Oil pump pickup can get fouled and screen is easy to scrub clean but I'm not sure it would cause noise.

Best of luck,

JohnMc