Thinking of buying used 555e backhoe-tractor, but engine has a loud knock ,it sounds like valve noise .the hrs meter shows just over 1500 hrs

You have to have your injector pump set at tdc by dealer or shop that deals with delco injector pumps. You set engine at 28 degrees btdc compression(marks will be on flywheel you can see through inspection hole) and then install pump tighten gear bolt then unlock injector pump,

Check the engine crankcase bolts have not come loose, i have seen this on some Briggs engines.

might be the valves have been damaged running without oil
- or the crankshaft / camshaft / main bearings
most probably you have to let the engine be rebuilt by a mechanic
(expect cost of more than US$ 2.000)
or get another (used) engine at a scrapyard

Sounds to me like your mechanic has it under control

It could be the compressor is going out, or the clutch itself is worn out. If the AC still gets cold, then it is the clutch.

this can be a serious problem, sounds like one of your engine valves, has the car overheated recently? if your valves bent, you need to take the engine apart hope i helped

It sound like you dropped valve seat on the head. Did you recently over heat your engine .

Well, not too many things can make a knocking sound from the engine. Unfortunatly, I would say that it sounds like you have a bad push rod. Did you buy the car new or used. It could have been a problem before you bought it(if it was a used car). There are ways to temp. stop the noise so that someone buying the car won't know there is a problem until later. I would like to know more about the situation so that I can thourghly help you find and fix this problem.

how is the oil level and pressure?, how many miles on engine? is this a loud knock or rather a clatter/ticking noise? knock is engine rod or crankshaft bearings, clatter loud ticking is valve follower noise. how long before it (the noise) goes away?