1966 Newport, 383, auto, stock except hei ignition. Fired up fine after long winter. Went for a joy ride a few days ago, now I have problems. Started it to move out of garage to wash windows, things OK. Tried to start to move back in, starter made the machine gun noise. Tested battery with meter, 12.5 volts. Put charger on. Battery up to 13.3v. Starter made same noise. Brought starter to parts store, tested good. Recharged battery, checked with load tester, passed. This morning, 12.7v at battery and same at large lead on relay. Tried jumping relay, same starter noise. I have 12.7 at battery, relay and starter. Goes down to 5-6v in key start position or jumping relay. Battery posts and cables clean. Any ideas? Thanks

Try bypassing each battery cable with a jumper cable.

Sounds like a poor connection at the battery terminal (either/both + and -) and at the engine termination points for both cables.

try another battery

Did the parts store test the battery under a load or just a "bench type test" The reason I ask is the starter may be electrically functioning but the drive clutch may be toast. That would not be apparent until a load was placed on the starter drive which could make the noise you're describing

Battery was tested under load, starter was tested to see if drive heat spun..

If the battery & starter are confirmed good & the cables/terminals (incl ground path) are good the only other thing I could think of is something fubared inside the eng greatly increasing drag (not likely I wouldn't think) or dist got bumped CCW greatly increasing the initial. I gotta Keep It Simple Sam & go with inadequate amperage reaching the starter (classic machine gun clicking). Can you set a known good battery by the LF wheel & jump it underneath directly to the starter with some jumper cables & see what happens. EDIT CW, missed that this is a BB

Your battery is bad.....

I agree sounds like battery.Mine went last year.Started perfect and went to cruise.Went to leave and pretty much nothing.Got it home charged and checked volts.Was at 13.Turn on key and try to start and pretty much 4 or 5 volts.New battery fixed it.Starter could be drawing a huge load though.But you would think if it drew enough amps to drop volts to 5 cables would get real hot and or smoke.Check cables also.Ive seen them look great but have corrosion inside.I run 1 gauge on mine.Rocky

Or the cables. They can fail internally and look OK on the outside.

all good advice. "green" cable internals are not that uncommon. all it takes is a tiny crack in the insulation, moisture, and time.
check your connections at ALL points.
if you have those "clamp on" battery cable ends, TOSS 'EM & get NEW cables ! i've fixed more electrical issues than you can believe, caused just by those cable ends !
try jumping the battery cable to solenoid at the starter, and the relay, after determining what voltage is present at the battery cable[s]. that may narrow your search.
electrical can be very frustrating at times, but we have confidence in you !
please keep us up to date on your progress.

Did they use a real load tester or did they use a digital tester? The digital testers do not put a load on the battery, they just check the internal resistance and spit out a number. They can be accurate but it is entirely possible for a battery to pass a digital test and fail an actual load test.

The severe voltage drop makes me think bad battery, or locked up motor.

Do a voltage drop test on the positive and negative cables. To check the positive cable attach one lead of your volt meter to the positive battery post and the other lead to the large post on the starter and read the voltage while trying to crank the engine, should be .3 (3/10) volts or less. To check the ground side place one lead on the negative battery post and the other on one of the starter mounting bolts and read the voltage while trying to crank the engine, again should be .3 (3/10) volts or less. If the voltage drop is higher than .3 volts you have bad cable or loose/corroded connections. If the cables are good you could measure the starter draw with a clamp on amp meter, should be around 200 amps. I think most likely you have a bad battery or cable/cable connection. Good luck.