Clicking noise

I have a 2006 Lincoln Zephyr and when I start the car some days the engine makes a clicking sound. Its not a constant clicking all the time, sometimes I dont hear it at all or it is very faint. It does rev up with the engine also. There is enough oil and good oil I use Castrol Titanium 5w20. I don't believe that on this car I can get the valves adjusted. I have 120,000 miles on the car, I bought the car brand new in 06. Can someone please give me so possible solutions please. Thank you in advance for all your help in this matter. Also I like to add that I recently had my oil pan gasket changed and he noticed so black residue around the cat. conv. he told me it was a real small whole, but being that there was residue around the hole that could be it. I hope that helps get me help :)