I'll try to describe the ticking/knocking sound as best I can so bear with me. First things first: it isn't engine related. The noise is there even when I put the truck in neutral and turn off the engine.

It is only audible when slowing down and then only from about 10 mph and slower. You hear it continually as you approach stop but its frequency decreases slower than the speed of the vehicle, meaning it is more rapid than would go along with the speed. BUT it does decrease with the decliining speed, just not as rapidly as the truck's speed itself. Does that make sense?

At first the sound reminded me of that of a tick in a speedometer cable but I don't think that is the case because it is still audible at such a slow speed that the speedometer needle isn't moving, and as noted, it isn't directly tied to vehcile's speed. It also isn't a rock in the tire tread, although it does kind of sound like that. Maybe a bit lower in pitch than that.

I THINK it is more on the left side than the right but I can't be sure. I am pretty sure it is getting progressively worse.

Any thoughts on things to check? By the way, I won't be doing this work myself but I did want some things about which to talk with the mechanic - whom I do trust completely.

Edit: It's a 2006 4wd with about 70,000 miles.
Thanks.

Check your CV axles. They will tick when they are starting to wear out.

If it a CV joint does that not also click or clunk harder when the truck is turned hard left or right. From prior experience with CV failures they usually show up easier if you drive a tight circle on either lock, it is the side you are using as the inside wheel that has the failure joint.

have you also checked the brake pads for sticking, maybe not sliding back evenly and this giving a slight bind noise.

I had a similar issue that randomly came up one day. I replaced the rear U joints and decided to do the forward ones as well and it turns out that one of the forward joints was bad but I couldn't tell with it attached. It sounded like it was one of the cv joints with the front jacked up but I couldn't really tell until I decided to do the Joints. It bumped up my gas mileage too.

Thanks for the replies folks. It ended up being a front U joint. MUCH cheaper!

Glad you found the problem and that the cost was not too bad.