I am hearing an intermittant ticking noise when the engine is hot. I don't think it is nailing. It varies along with the RPMs. The noise usually gets louder the longer I've been driving. But the noise does not always occur. This morning on the way to work I didn't hear it at all. It sounds like it is coming from the front of the engine somewhere. At first I thought it was coming from the front injectors, but it's hard to locate the noise, and I don't have a stethescope. I wonder if it has something to do with the fan clutch, or some other cooling system part that would be activated when the engine is hot? Any ideas?

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine