Engine making a knocking noise?

Thinking about buying an XJ, but it has a problem. Whenever jeep is started or when the gas pedal is stomped to the floor, the engine makes a knocking noise. Owner said it might be an oil pump shaft, but i was wondering if it could be something severe, such as a broken rod. Has anybody ever had similar symptoms? Is there a definite way to tell? Thanks

can you give anymore info on the Jeep?
year, engine, milege, tranny.
have you heard the noise, or is he describing it to you?
if your serious about buying bring it too a reliable mechanic you know or trust and let him have a look at it. $50 for him to inspect it may be worth $200+ off the asking price if you can tell the owner what the problem is and what it will take too fix it.

if its the oil pump shaft, youll feel it in the distributor cap when you hold your hand on it.

if it is a broken rod you will not have to hold the pedal to the floor. You would start it and want to shut it down right away.

Knocking in my motor I swore was a valve tap once I got it apart it turnned out to be just a broken exhaust manifold.

On my 90 4.0 it would knock at startup and when idling. Turned out to be a cracked flexplate(automatic flywheel). The flexplate cost about $60 and they wanted 450.00 to install it.

Beakie, it is a 92 Laredo 4.0 with 142,000 miles. Tranny is an AW4 automatic with a 231 TC. I have not driven it but owner described the noise to me. I might drive it soon and check it out. I wish i could get price dropped for problems, but a cherokee for $600 is expected to have some problems, and i dont think he wil be going any lower. Also, this jeep competed in the Uwharrie 4x4 cross a couple months ago, and came in 2nd place in A class, so im guessing it has been pushed pretty hard.

Thats how much I paid for mine.....

As for the engine noise, How deep of a knock??

Is it like knocking on a solid wood door (thud), or does it sound like a diesel engine(rattle)?

Solid door thud, is most likely main bearing failure and would result in low oil pressure at idle.

Diesel engine would be heard at above idle and would indicate a connecting rod bearing failure (this could also be piston slap, which is fairly common).

Both of the above warrant an engine teardown and possible rebuild......

If it's light ticking, its probably a lifter.

if it's a SUPER light barely audible tick, it's injectors.....