2001 Toyota Corolla clicking engine, loss of oil, dark? Oil

I have been noticing an increasing amount of engine clicking in the last 20,000 miles. I notice it most when decelerating from 70mph (most likely other speeds also, but not that I can recall at the moment.)

The engine has also been loosing oil, perhaps a quart a week. And the oil when I check it is dark. I try to get the oil changed every 3000 because of the high miles.

Where should I get it checked at? (Major store chains)
What type of checking should be done?
And what repair recommendations should I be suspicious of?

I just had the timing belt (major cracking) and front axle replaced (broken).

Thanks