Ticking in M103... LOUD ticking

Hi folks,
I've got an '86 300E (230km) that has developed, over the past few months, a distinct (read: loud) ticking noise from the front of the engine, it seems. It sounds somewhat metallic, increases with revs (in freq and volume), and is persistent. It also seems to cycle - eg., at idle, it gradually increases in volume for about 9 seconds, suddenly drops off, and then immediately starts up again. It is more pronounced during colder weather, but really will appear most anytime. Sometimes goes away, but it has recently been there about 90% of my driving hours. The car is well-maintained (fresh Mobil 1 regularly, etc.)
Any ideas?
Regards,
-Nick