can you describe this ERRRRR is greater detail? like a groan sound or a screech? as steven stated, itll crank really slow. or if it is fubard with sit there clicking away. As well sometimes when it is starting to go it will crank normally but at the end it almost sounds like the solenoid gets stuck and it will grind like you over started it...even though you didnt.
Have a feeling the ERRRR you described is not the starter
BUT i was just looking through this and this is the process of testing the starter at home
To test it at home first inspect the pinion gear and the ring gear on the engine where the starter was attached. Look for severely damaged teeth. If there is damage, replace the starter or flywheel. Use your battery charger or jumper cables and a car battery. Hold the starter on the floor with your foot. Connect the negative cable to the body of the starter. The flange where it bolts to the engine of transmission works well, now touch positive to where the wire enters the motor body. (NOTE: Be careful to NOT touch the starter body. Doing so may cause a fire. As usual, I am not responsible for anything.) The starter pinion gear should sprig forward and spin. (If you have a cheap, low current charger, it may overload. Starters consume LOTS of power.) If the starter works, several consecutive times, it is good. Sometimes, however the problem is intermittent."
came from the thread https://www.rx7club.com/1st-generation-specific-1979-1985-18/end-all-no-start-threads-warning-book-inside-162062/
which may help for further diagnosi