if it makes you feel any better, my 237 has had a rod bearing knock for several years. it is the number 2 rod bearing, and i replaced it twice. the first time with the same size insert as on the other 5, the second time with a slightly oversize insert. in both cases, the knock went away for about 250 miles, but has returned. either i'll pull the engine and take the crank to a machine shop to regrind the rod journals, or i'll wait for the engine to put a rod through the side of the block and pull the engine and replace it with the 251 spare i have sitting on a crate in the garage.
in all, i've put nearly 1,200 miles on it with the knock.