Exhaust leak is my guess
Several ways you can check for exhaust leaks.
Visually check for carbon marks around the exhaust manifold
Spray soapy water on a cold exhaust manifold right at start up and look for bubbles
With car off blow air into the exhaust pipe (hair dryer or shop vac on reverse)...
I think they sell a polarity independent LED flasher now, of the same type:
https://www.amazon.com/EF32RLNP-Electronic-Compatible-APPROVED-INDEPENDENT/dp/B01GIU9M0Y/ref=sr_1_6?ie=UTF8&qid=1511575264&sr=8-6&keywords=led+flasher