I have a 97 land rover discovery 4.0. When running the a\c for awhile driving down the road i will come to a stop, once i am sitting there for a second i get a load knocking noise under the hood (sounds like the rods) but when i cut the a\c off it will go away! it doesnt do it all the time and will not do it unless the a\c is on. Its not my compressor but does sound like its coming from the bottom of the motor as best as i can tell, i dont think its a rod knocking because it only does it with a\c on while sitting still when the motor is hot or running for awhile, anyone ever had a problem like this?????

 

Yes it could be a rod knock, with the a/c on the entire engine gets really hot from the extra heat and that includes your motor oil.
If the oil gets to thin it wont do its job.
What oil do you have in it now?
Weight, type and brand.
When was it last changed and who changed it?

 

Thanks for the help. It has done this threw 3 oil changes, i have mobil 10w40 in it now, with stp oil treat. Its weird because i can drive it all day long and it will not give me a problem, i can let it idle for hours with no knocking noise, but when i turn the a\c on (while sitting still) it will start to make the knocking noise just for about 10 seconds then go away, but will not do it driving down the road. Im going to get it to do it again a crawl up under and and start trying to find it!!

 

If it is only with the a/c on then I have to say it is the a/c, or maybe one of the aux cooling fans is loose and knocks when it first starts up.
You will need to start poking around to find out.
I dont think it is actual rod knock because it has not blown up yet.
It could also be a exhaust rattle of some kind.