Cold start isn't loud?
 

I have a 2008 lexus is350 with custom exhaust and recently when it starts to drop to freezing temperatures (31 f) or below my car's cold start each morning would sound like a normal warm start.

But after I start it up to run my errands for the day, I come home and go inside for about 2-3 hours then when I start it up again that's when I get the loud obnoxious cold start sound.

Any ideas? maybe bad spark plugs or who knows.