2003 Tundra started to beep while parked but its not a series of beeps. Its one long beep.Drove to work and everything seems fine but loud noise continues. It is coming somewhere from under the hood.

I had the same problem on my 2003 toyota tundra, loud beeping sound continuously, even after I lock the truck and parked in the drive way.
So, I did a search and there are multiple possibilities such as weak remote key battery, bad lock sensor, abs sensor, etc. but my oem obd2 reader didn't show anything. So, I found the speaker and I disconnected it. I found it between the battery and the windshield.

Could it be overheating and what you are hearing is the radiator cap blowing off steam