Slow cranking moter will start sometimes othertimes starter clicks

I call that chatter and means the starter solenoid
sees only 8vdc from the battery , cranked.
there are 6 causes, due to 6 things(parts) make cranking go.
why do all that,and not do jumper cable jump starts first. if works then battery is NO GOOD>in car.

posted this 100s times.
both cable ends (x4) not loose or rusty.to bat.
1: battery good means alternator not bad too.
load test the battery or jump cable jump it.
we know the starter is good, last words of your post.
2: then key switch good, and ST pin sends 12vdc
to the PRNDL switch.
3:PRNDL on/in the tranny must work in park and neutral both so all techs and tow truck mean test neutral cranking next or gets egg in face.
4: next up (it is a series path of DC ST )
the starter relay closes, called energizes.
and sends 12vdc to the soleniod and starter cranks.
5: starter bad , no it is not.
6: wires bad.(and or connectors)
did not jump start your car
did not charge battery and load test it. oops

comments:
NO 15 year old battery is good, my 50 year record is 10 years life, (and not common)
did not crank in N neutral PRNDL.
did hot wire starter and starter is loaded for bear (good)
and wires. yes can fail #6
so all techs know that and out pops Mr./ DMM set to volts to find where the path fails easy.
normal cranking voltage is 11vdc.
the 1 volt loss is battery internal resistance at 300amps load.
8vdc the solenoid chatters like mad. a huge key fact.
it only takes a voltmeter to find this failure.
after jumper cable jumps fail,
if not fail Jumped, the battery is bad,(or rusty lugs)

good luck , be happy for 6 and not 50 engine failures and lots of tests. that.

we charge the battery first then load test it , a tool..
or have shop test your battery ,even walked in.
or find a good battery out of 2nd good car. TRY IT.
or jumper cable jump start your car off 2nd running car just like AAA Auto club tow trunks do endlessly.
day by day.
or the voltmeter across battery terms (clean) show 8vdc cranking is a bad battery.

not withstanding driving car the battery dicharged dead , alternator dead.

The clicking noise will be relays. Possibly a starter relay and thats normal. Sounds as if the starter moter may be the culprit. You have to check for start power at the starter solenoid while cranking before any further daignostics can be done. Sometimes tapping on the starter motor while holding the key in the crank position will start the vehicle. If so the stater motor is defective

Probably time for a new battery. If it is 3-4 years old or older I would lean toward that. Take the truck to your local parts store and have them test the battery out.

I would say if the car stays running after u jumped started it I would replace the battery

The solenoid on the starter may need to be replaced.

a worn out starter could do it if the bushing in it are worn it would pull to much power from the battery and would not turn the moter over fast enough to pump enough gas to start the car if its carberated then it has a manual fuel pump the fuel pump wont pump enough gas if the moter is not turning over fast enough i would either rebuild the starter or replace it.

if your connections are all ok and the battery isn't VERY dead and you are using quality jumper cables ....you may have a bad starter motor.

Lights dimming a good clue, that means that there is power going to the solonoid but starter moter wont turn. Check all battery connections and starter connections Make sure battery has enough power to turn starter or try jumper cablea.
Sometimes the starter gets jambed up due to poor lube on the bendix. try hitting the housing with a hammer. You can take the starter out and bench rebuild or just replace it its not a hard job,
\

My truck was doing the same thing, and it turned out the starter was going bad, it started drawing too much juice from the battery, so the crank power from the battery wasn't enough to start the motor, but the battery was good, if you jump start it from another vehicle and it starts right away, it is most likely your starter.

check the igination switch.
do you get power to the starter???
you need to check first.