monte has a whistle
 

04 ls 3.4, did thermostat and then it wouldn't start, would immediately shut off. Unplugged MAFS and now the car stays running but whistles loudly and revs high. I'm lost. Any ideas

Sounds like the MAF could be messed up. The whistling could be a vacuum leak.

Yeah it turned out to be a leak in a hose. Got that fixed and the thing still overheats and blows out nothing but cold air. Thinking heater core?

Overheating could be from air in the coolant system, as could the heat issue. Try bleeding the coolant (of course adding antifreeze if need be), but it wouldn't surprise me if the heater core was failing - or even the water pump or an intake gasket leak.

Has it been using/leaking antifreeze?
If so,
I'd have to agree.. could possibly be bad intake gaskets.. and with air in the system.. would be a good reason that you don't have any heat.