It could be perfectly normal, and nothing is going bad. If you are really concerned, then get a mechanical oil pressure gauge, and connect to the sending unit. Read your oil pressure cold at idle, and hot at idle and the engine running at, say, 2000rpms. Record the numbers and post them. If your idle pressure is below 10psi, hot, I'd consider you may have wear problems. If it is above that, and 2k pressure is above 30, I'd say you should be OK. If you had a warning light for oil pressure, it would be factory set to illuminate at about the 5-8 psi range. They are not really too accurate.
If you cannot develop pressure, with a low mileage engine you'd look for something abnormal, with a high mileage engine, you'd expect and look for bearing wear.
tom
added: It sounds like injector firing to me, or possibly slight valve train noise. The consistency makes me think it is normal noise. Some injectors are noisy. The Mazda 2.3 did have problems with the intake manifold runner flapper thingy.(tech term!!) It would make noise that would drive a saint crazy. Disconnect the vacuum line, and wham, quiet.
splain: when I say Mazda, I mean the engine is a Mazda design, slated for a front drive passenger car, and turned for truck duty.
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