Thank you all for a great suggestions.
We are checking if the issue is resolved.
This is what we did:
1. replaced relay board - initial beeping eliminated during pre-heat. As soon as it pre-heated and temp going down, long - short beeps are continues.
2. exchanged heat sensors between lower and top. (did not installed new) - issue is completely resolved. Both units are working normally without beeping. (mystery) However, when running both ovens and turning them completely off, got "Err" displayed. No number, just "Err". It happened only 1 time and we cannot re-produce it again. Will be checking with a customer after a while.
- Samurai Appliance Repair Man likes this