1987 Mazda Truck Ticking Noise

Engine problem
1987 Mazda Truck 4 cyl Four Wheel Drive Manual 165K miles

This truck has the non-fuel injected 2.6 liter engine. It sounds like a lifter but since this is an overhead cam engine, I know there are no lifters. I believe there are hydraulic lash adjusters but don't know if they could cause this issue.

The ticking is most pronounced when the season is cold. During this time, the tick is constant but does lessen as the engine warms up. I've tried changing oil viscosities and have noticed a difference but it doesn't go away. I've tried 10W-30, 10W-40, and currently using straight 30W. This last viscosity seems to perform the best and is less noisy. During the summer months, the noise is present during initial warmup but then will go away completely. For the most part. I do hear an occasional tick or two while idling.

Don't know if this matters but the previous owner "de-smogged" the engine and installed a webber carburetor. I'm not fond of this but otherwise, the truck runs well. Also, I have tried oil additives (Power Punch, Marvels Mystery Oil, added a quart of auto trans fluid to clean out what might be lodged somewhere. It was a long shot) but nothing seems to make a difference.

I considered the possibility that the noise might be an exhaust leak from either the exhaust manifold or where the exhaust pipe connects to the manifold. Other than trying to feel around the pipe while idling, I don't know how to check for this. This inspection didn't reveal anything.

I would be interested in a professional opinion and ideas for correcting this. Thank you.