V6 3.8 99 trembles

[Reaper 0-1]

I rebuilt my 1999 V6 Mustang, when I obtained the car she was in bad shape. The head gasket eventually blew so I ended up pulling out the engine and rebuilding it. After putting it back together, it sounds great and it's a smooth ride, but there's this tremble. It only happens when I come to a complete stop and my foot is on the brake. The tremble happens every min or so for 2-3 seconds. So did I do something wrong? or have I overlooked something?

 

[Phil II]

Could be a lot of things. A video might go a long way to helping diagnosis.

Is it an auto or manual transmission?

 

[Reaper 0-1]

Could be a lot of things. A video might go a long way to helping diagnosis.

Is it an auto or manual transmission?

It's automatic.

 

[Flex]

Try pulling codes. By "tremble" do you mean the car shakes?

 

[Gigs86]

Sounds like maybe a vacuum/ exhaust leak... My car sometimes does it, I know my right header is leaking where it mounts to the down pipe. Haven't pulled it off yet to fix it.