Check the engine oil level first. It is hard to guess at a noise without being able to hear it first hand. It could be a hydraulic valve lifter or valve that is out of adjustment, it could be a more serious problem due to oil starvation or lack of lubrication to an internal engine part, rod bearing perhaps.
A friend of mine heard a noise he thought was very serious in his Dodge Durango, he called a tow truck to get it towed to the dealer, the tow truck driver started the engine and told him not to bring it to the dealer that the engine had blown (suspected crankshaft damage the engine was making so much noise). He towed it home, I looked at it, it ended up being a fan belt that had shredded and was thrashing around in the engine compartment. Get it checked as soon as you can it may be something minor!