Tapping noise from valve area in 1990's Toyota Corolla

Question: Did installing a rebuilt starter cause a tapping noise in the engine that wasn’t there before? And why did the noise eventually stop after driving the car 5 miles? Is it related to fuel accumulating in the intake manifold from the cold start valve, part of testing the starter? Or did losing a cup of coolant from the heater hose create an air pocket which caused the noise?

Recent History: I installed a rebuilt starter motor without incident, other than I got busy with another project and the car sat in the driveway for three weeks before I got it done. Normally I drive the car daily.

The problem: The car started up normally after the starter installation was completed. I drove the car around the block and thought it didn’t sound quite like before. Also, it seemed to be running at a higher rpm. I parked it back in the driveway and let it idle while I listened to the engine. As the engine temperature rose to the normal operating temp, the electric fan tripped on as expected. About a minute later, I heard this sort of tapping noise starting. Noise was coming from the top of the engine, the valve cover area closest to the firewall, about midway down the valve train. Difficult to say exactly where it is coming from of course. It could have been coming down closer to where to the intake manifold meets the head. Or it could have been coming from where the exhaust manifold meets the head. I used my stethoscope, but hard to tell.

When I increased the rpm (by opening the throttle, the car in neutral and parked), the noise became considerably louder, and had a sort of “pop” to it instead of a “tap” . And it’s frequency increased with the rpm. This noise was very noticeable and wasn’t there before the starter replacement. In fact the car has ran like a top since it was new.

I checked the engine oil and the coolant levels. Ok. All guages nominal. Not overheating.

I looked over the engine compartment and couldn’t see anything out of place or hoses disconnected, etc. So I got back in the car and took it for a drive. I opened the throttle and goosed it a bit too. I’m thinking there is some kind of ghost in there that needs exorcisng. About 5 miles. When I got back and parked the car, the noise was gone. And has never come back, at least yet. Any ideas?

I’m thinking one of three things:

  1. Parking the car for three weeks caused something to sieze up, and whatever it was, just driving it caused it to free up.

  2. As part of installing the starter, I cranked it a dozen times or so with the ignition disabled. But I could hear the fuel pump come on. I’m assuming cold-start valve was shooting gasoline into the intake manifold. Perhaps this accumulation of gasoline was the problem?

  3. Also as part of installing the starter, I had to disconnect a heater hose. About a cup of coolant came out in the process. I figured I’d drive the car for a week and then top off the coolant. But it may be the case that an air pocket was introduced into the coolant system. Maybe that was the cause of the noise?

  4. There’s something else wrong that has nothing to do with the starter or it’s installation, and I’ll just have to wait until it comes back.

Anyway, all ideas welcome. Thanks.