With out compression test or leak down test the number 3 cylinder problem is most like a lack of compression. Poor burn will make odd noises from a low compression.

There's also the possibility of a faulty injector which can be determined by swapping it with #1 and see if the noise follows the injector.

Pumps do have faults but rare. The idle control system on the 126 /603's will make additional noise as it tries to stabilize the idle speed.

Up under the drivers side firewall is a round cap with numbers on it.....turn the cap to the highest number and see if the noise is reduced.



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i'll see if i can find somebody with a comp gauge for diesels and see what i got.