I looked/searched and couldn't find anything for this specific issue- feel free to redirect me if I missed it. I'm hoping someone has seen or heard of this before and can at least help me narrow down where to look so I can track it down. It's odd.

My 1991 300D, OM602, 130k, runs beautifully, but has a very precise, periodic tick that cycles at nearly perfect 18-second intervals. From the moment the tick becomes barely audible, it spends almost exactly 10 seconds building in volume/intensity, then goes silent for 8 seconds

Stick with me here, I know it's sometimes futile to describe sounds over 'net, but I'm giving it a go anyway: So it's not audible, becomes softly audible, then "tick-tick-tick-tick-Tick-Tick-Tick-Tick-TICK-TICK-TICK-TICK-TAK-TAK-TAK-TAK-TAK," then suddenly silent again for 8 seconds. The interval is about 3-per-second at idle. After the 8-second silence, the noise builds again for 10 seconds, goes silent, and so on. Happens cold or hot. I'm clueless

If it were an injector, or maybe valve noise, it would be pretty much constant, correct? What the heck would cause a periodic cycling knock/tick?