what is causing the ticking?

You have "pre-injition" Basicly a carbon build up. You need to start using the next grade in fuel. You might want to go in for a "injector service". As cars age this happens. After the injector service I would recommend that you change your plugs. And this will cure the "ticking". If that is not the problem, You could have a exhaust manifold leak, that is common. You might need new bolts. Or if that dont work, You could also have a bad spark plug wire. All these will cause "ticking" upon acceleration. I would recommend that you get it looked at, the mechanic will be able to direct you on which issue it is. Best of luck.

In addition to all of these things, you may want to look at the valves. The rocker arms can loosen up and allow a bit of freeplay which will lightly "tick" as rpms increase. I removed the valve cover on a 1992 Jeep Cherokee and ran the engine as I tightened each valve slightly until the ticking stopped to fix a prob. just like this.

Rare source of ticking noise from under a Tahoe 2003, initially I only noticed it with acceleration. The ticking sound came from under the vehicle, not from under the hood so much. Occurred with acceleration from a stop, couldn't hear it after about 40 mph. The ticking was amplified when it bouncing off a wall on the passenger's side with passenger passenger window open, again, with accelaration. I got out, listed from outside as my son started out from a stop. The ticking sound came from under the vehicle, and as best as I could tell, seemed to come from the area of the bell housing. Then more clues. Accelerating in reverse did NOT cause the ticking. BUT braking with a little force while backing up did indeed cause the clicking. Also Idling on a steep uphill slope, or moving slowly up a steep uphill at a low speed also caused the clicking. Idling on a steep down slope did not cause the clicking. It was a momentum and gravity thing. The retaining spring for the starter gear, or whatever keeps the starter gear from engaging the fly wheel when the engine is running was broken. Therefore the starter gear would slide backward or forwards on its own. Acceleration caused it to fall backwards into the fly wheel. Braking in reverse also caused it to fall backward into the flywheel. Got a new starter, the tick is gone. DrD