Making big noise does not want to stop - 1990 Pontiac Bonneville

It’s difficult to say because engines make a wide variety of noises.
Rattling, possibly a worn timing chain.
Misfiring, possibly incorrect valve timing or damaged head gasket.
Metallic scraping, possibly a lack of oil pressure causing bearing to run dry.
Knocking, possibly worn mains or big end bearings.
Loud exhaust noise, possibly a blown exhaust manifold gasket.

There are a wide variety of possible noises and therefore a wide variety of possible causes

Yes that sounds right. BIG dollars to repair.

Unplug the IAC. Does the noise go away? If so you probably have a bad PCM (computer) and IAC. Possibly bad wiring to IAC. IF noise does not go away unplug starter, does noise go away?

This is normally a sign of a leaking exhaust valve, it is too tight. the popping is unburned fuel leaking into the exhaust and then exploding with the exhaust from the next firing, sorry but a big job
Merry Christmas

you must replace the timing belt. Otherwise you must stop the engine and take another BIG problem. DO IT NOW!
God bless you

It could be a "big deal" or it could be relatively minor. Since you can't be sure before you buy make sure you offer low enough to pay for a power steering pump replacement - $400-$650 at the dealer. And the pump itself sells for $80- $272 if you replace it yourself - it is not that hard either.

blend door on heater is bad , big job to fix

not a big problem if your handy its the regulator and motor you can get it on ebay for a good price! waiting for ups to del 1 know.got the same problem with the 95 929 that i just got .1 window was stuck in the back and the front drivers side makes a loud noise. got 2 of them for 110 bucks and put them in myself saved about 300 in labor good deal

replace u-joints

volvo s80 y reg 2.4 runs ok little bit lumpy tickoover when warm but very loud whistling noise take dipstick it stops but very big suction from tube not pressure it is not breather box under manifold