It's all new to me, never had a car do that before.  I have moved from Florida to New Mexico.  The elevation in Florida was 26' above sea level, here in New Mexico my elevation is 7300' above sea level.  The car is not running good right now.  I suspect a fuel filter and perhaps a carburetor adjustment.  The car kept dying at stoplights. 

That does seem odd. I have the 24" on my car and they are not any louder at start up than at operating temperature.

Durham Missouri

Well I installed the 30" smithy's and when I started the car, it was extremely loud.  It had a rapping sound that would wake the dead.  I drove the car a few miles and the car quietened down and sounded wonderful.  Now every time I first start the car, it's extremely loud and then quietens down.  Could it have something to do with fuel delivery during warm up. When the car gets up to operating temperature, it sounds great.  
The old Smithy's did not do this.  

I looked under the car a while ago, it has the 30 inch Smitty's.  That is what I will buy.  Mine have pinholes in them.

Do you have 30 inch Smitty's or the shorter mufflers. 

Smithy's get my vote. I have a set on my car and they sound great. Nice thump at idle and not too loud on the highway. And this is on a 300ish hp y block with a fat cam.

Durham Missouri

Thanks everyone for your reply's.

I wanted to get the longest set of Smittys, 30 inch long I believe.  The muffler guy said he couldn't weld them in.  I may just buy the
longest Smitty I can get  and find a different muffler shop.  The Thrush turbo 17715 sounds like a good compromise and they are rated fairly high.
I talked to Mr. Brockman of muffler fame and he said Y Blocks are loud motors.  I bought his mufflers and removed them 15 minutes later.  The were beyond loud.

I know the feeling too many years on heavy equipment, airboats, motorcycles, and shooting firearms. In my day, no one wore hearing protection.

Back in the day, we always put on the cheapest glass packs we could find.  We would run straight pipes if the law would leave us alone.  To pass inspection, we had our girl friends drive the car through the line.  They always passed.