Help! Engine clicking while running!

I just started up my car after filling up gas and my engine started clicking. The clicking is directly proportional to engine speed. I did the math and I get about one click per RPM.

I've been having trouble with a pulley lately screeching in a narrow RPM band of about 3000-3200 so maybe something happened with that but the click sounds like it's coming from the block, not near the pulleys.

Right before the clicking on the way to the gas station it was running pretty rough with a weird "chug" at about the same rate as it now clicks.

Could it be a bad spark plug? Fuel injector? The car has never failed an engine balance test and I haven't gotten any codes in weeks.

Any help would be great, I have to drive cross-country in a few days.

Thanks!

I've read that bad lifters and valves can cause clicking.

My oil was also running low so last week I filled it up to the max on the dipstick and a few days later (the day before the clicking) my oil pressue gauge stopped working after a few heavy acceleration on-ramp runs. I've heard a bad oil change and non-optimal oil pressue can screw with the valves and lifters.

Got an update.

Couldn't figure it out myself, took it to a mechanic, oil pump went. The clicking was my unlubricated valves.