A month or so ago I installed an old (circa 2000) Flyin Miata kit onto my car.
If I park the car at operating temp for a short amount of time (for example, filling the gas tank), when I restart the car I get a rumbling feeling and sound near the pedal box.

My guess/assumption is that the turbo manifold was heating up the heater core lines going into the firewall. That coolant was boiling gets circulated through the heater core once the water pump starts spinning.

I haven't had any issues associated with this, but it isn't confidence inspiring. I couldn't find any threads about someone experiencing a similar issue.
I built a heat shield for the turbo and have wrapped the coolant lines in some insulation, but it hasn't helped.

Hoping to get some thoughts and opinions about this.
Thanks