1) draft diverted plugged/missing, exhaust replaced recently? Modified?
2) case to HE flange in vestibule leaky. The seam between the two may be bad.
Watch flame of main burner as fan turns on. If flame is disrupted then HE failure is likely. Look in blower housing for weird rings. If you find them (in blower housing under squerill cage) then HE has failed. Look at the seam for the heat exchanger where the burners enter. There is some surface that is common to vestibule, burner, and HE. This should have a gasket that may have failed.
Describe type of HE ie: barrel, s type etc. pictures are nice too.
I think you will see the main flame disturbed when blower starts and that USUALLY means failed HE.
Intermittent kit may solve it, or rather may avoid the issue.
You can also try a combustion analyzer and watch for co2 or co changes as blower starts (bad HE)
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