Sync with google is enabled in system settings, and inside gmail, set to sync last 4 days. If i open gmail, press update and then leave gmail, a notification will beep if there's new mail(s). Has been like this since nightly 28 to nightly 58(current)...
Sent from my LG-P970 using XDA
Could be a data corruption problem somewhere. Try:
1. Nandroid backup
2. Factory reset/data wipe
3. Check if the problem persists after a clean install.
If it works ok, selectivly restore your apps a few at a time to try and find a conflict. If it still works after restoring all your apps, it was probably a data corruption.
If it doesnt work after a clean install, nandroid restore. If that happens, i've got not idea what the problem is.