There is a ticking sound
In order to give you a solution to your problem, I need to explain a little bit about the valve train in your engine, the engine itself, and how it works.
1.You have an Internal Combustion engine. It is a Four Stroke engine. The engine has a Cylinder Block with cylinders inside. There is a piston for each cylinder which goes up, and down. The piston/s are connected to a crankshaft. The crankshaft turns the transmission, which in turn turns the driveshaft, to the rear differential. The rear differential has axles, which the rear wheels are bolted to. The four strokes are , Intake Stroke, Compression Stroke, Combustion Stroke, and Exhaust Stroke.
The piston goes down the cylinder drawing the fuel/air mixture in. (Intake Stroke) The Intake valve opens. The piston comes back up the cylinder, and Compresses the fuel/air mixture. (Compression Stroke) Both the Intake and Exhaust valve are closed. The spark plug fires igniting the fuel/air mixture, and shoves the piston down. (Combustion Stroke) Finally the Exhaust valve opens, and expels the burnt gases. (Exhaust Stroke)
This page on Wikipedia.org, may help explain the process. The third 'photo' down on the right is an animation showing the process.
The animation shows an engine with an Overhead Camshaft. Your camshaft is located in the Cylinder Block, and not in the Cylinder Head, as shown.
2.Your Camshaft is a shaft with egg shaped lobes on it. As the tip of the egg shape comes to the top, it pushes up on a Hydraulic Lifter. This lifter in turn pushes up on a Pushrod, which pushes up on a Rocker Arm. The Rocker Arm in turn pushes down on either the Intake Valve, or the Exhaust Valve, opening them.
A Rocker Arm is shaped a lot like a See-saw. Just like the one's at a child's playground. As one side goes up, the other side comes down. The Pushrod pushes up on one side of the Rocker Arm, and the other side of the Rocker Arm pushes down on the valve, opening it.
A Hydraulic Lifter is a small cylinder that has a piston in it. Oil goes through a tiny hole in the side of the lifter, and this keeps the piston in a certain position. (That's why this lifter is named 'Hydraulic', because it uses oil inside) The Pushrod rests on this piston. The hydraulic action of the Hydraulic Lifter, keeps slack out of the valve train. The pushrod to rocker arm distance, and the rocker arm to valve distance.
What you are hearing, is a clicking sound from clearance being created, in-between the pushrod to rocker arm, and/or rocker arm to valve stem. A metal to metal clicking sound.
Solution? Depends on how mechanically inclined you are, or you may want to refer this job to an auto repair shop. The valve cover needs to be removed, and the nut on each rocker arm needs to be adjusted. Adjusting the nut down, (Clockwise), pushes the rocker arm down on the rocker arm stud, a little. This removes the slack, and makes things nice, and quiet again. Your engine will also run better, and you'll get better gas mileage.
DON'T do this, or have it down, and eventually the slack that is in there will increase. This will break parts! There IS a technique in doing this. If you know of someone who is good at adjusting valves, they can do it. They MUST be good however. If you adjust the Rocker Arm Nut too far down, you will lose power, and the exhaust valve face will burn. Not far enough down, and you get the clicking sound you hear now. About 1/4 turn down, to 1/2 turn down, should do it.