Rapping at startup

I have a 2000 xj (4.0) with about 95,000 miles on it and i use 10w-30. It will rap for a few seconds at startup. I just started doing this the last couple oil changes. I was using Mobile oil and when they changed to the extended life it started rapping at startup after I hit about 4000 miles. I would change it and it would be fine till about 4000 again. Then I switched to regular Castrol and changed it every 3,000 and didnt have a problem for the last two oil changes. then I had about 1,500 on it and it does it sometimes. usually first thing in the morning. The oil pressure is not high ever but it is over 13psi at idle and over 37psi at 1600 rpm. Which i believe are the factory specs. But it never goes over 50 psi ever.

The rapping sounded exactly like no oil pressure like when you first change the oil and the filter is not full yet. The oil pan was leaking a little so I decided to change the oil pump. The oil pressure is much better when cold. However after driving 10 minutes it drops just below 20 at idle. After about 2000 miles it really starts to drop more. So the last change I put 10w-40 in it. Its still about the same as the new oil after the oil pump change. My coolant drops alittle but the thermostat housing leaks and I haven't got around to changing the gasket yet.

Any Ideas why the oil pressure drops so much over the life of the oil?

What oil filter ? Please let it NOT be a Fram.The oil you are using is OK ,i only run a thicker oil(15w40) in the summer months,10w30 when the temps drop.My 96 now has over 240,000 and has normal(per spec) oil pressure.
Wayne

ha ha ha trust me I know better that to use fram. I wouldn't put one on a go cart. Its not where the pressure is but the fact of it rapping at startup that bothers me. It sounds really really bad.

Well then it sounds like the oil is draining out of the filter,maybe bad filter of the brand you do use.Try changing filter brands,I use puroilater,or wix,NAPA gold which is the same.
Wayne

I have tried several different brands. Bosch, STP, Mobile. They all do the same thing. Its only after some miles on the oil. When its fresh its normal. Its after 1500-2000 miles iot starts to do this. I hate having to change the oil at 2000 miles all the time.

I do have another idea. What else could cause the oil to slowely degrade like that. The colant slowly disapears but I always assumed it the the leak in the thermostate housing(I know it is leaking there). If the head gasket has a small leak would it cause the oil to slowely loose its viscosity? The oil always looks mormal. Never milky or stangle in anyway. But could trace amount of colant hurt the oil ?

Do a search on cracked heads .There is alot of info on the problem in your year XJ.To answer your question a little ant-freeze in the oil over a long period can cause bearing problems.
Wayne