Usually: Not enough current reaching the starter — usually the battery or its connections.
But it can also be: Corroded terminals, a loose earth strap, or a failing starter solenoid.
Rapid Clicking When You Turn the Key
Turn the key and instead of the engine cranking over, you get a fast, chattering click — like a small relay stuttering. This is almost always a current problem, not a broken starter. The starter motor is asking for more power than it's getting, so it engages and disengages over and over instead of turning smoothly. Most of the time that comes down to the battery or the connections feeding it, not the starter itself.
Here's a rough way to tell what's going on before you spend any money. Pop the hood and look at the battery terminals. If you see white, blue, or greenish crust built up around them, that's corrosion, and it's a common, cheap cause of exactly this noise. Try wiggling the cables at the terminals — if the clicking changes or the connection feels loose, you've likely found it. You can also try turning on the headlights before you crank the engine. If they're dim or barely glow, that points to a weak battery or a bad ground strap rather than the starter. A single solid clunk, versus this fast chatter, points somewhere else entirely — but rapid clicking specifically is a voltage symptom nine times out of ten.
Is it safe to keep driving? The car isn't dangerous sitting in the driveway not starting, but you don't want to be stuck somewhere unexpected. If it's clicking but occasionally starts anyway, treat that as borrowed time — the connection or the battery is getting worse, not better. Don't keep cranking it repeatedly; that can draw down a marginal battery further or stress a solenoid that's already struggling.
Fixing it generally starts with the cheap stuff first: cleaning corroded terminals, tightening or replacing a loose ground strap, and testing the battery itself under load, since a battery can read fine sitting still and still fail to deliver enough current when asked. If all of that checks out and the clicking persists, the next suspect is the starter solenoid, which is a more involved job since it usually means pulling the starter to test or replace it. We won't know which of these it is without actually testing the charging system and connections — we don't recommend guessing and replacing the starter first, since that's the more expensive part and often not the actual problem.
If you want to bring it in, we can test the battery, alternator output, and connections on the spot and tell you which one it actually is before you pay for parts you may not need.