it will be probably have been set to warn of Points of Interest. These can identify petrol stations, Weatherspoons pubs,
Screwfix shops, traffic cameras, railway preservation societies, whatever you are interested in, and make an alert. You can download lots of POI lists but AFAIK you need the PC installation disk to get their pacakage which does it. It is called TomTom Home. Maybe you can get it off the net. My Tomtom has a mini USB socket underneath for connecting to a computer.
You can choose what POIs you are interested in, and you can choose what noises to make.
I have not got mine with me at the moment, you you will want to poke the screen and navigate left and right by touching the arrows, until you find an option like "change Settings" or "Manage POIs" then you can see what POIs it is set to, and change, remove them, or change the sound.
If its not POIs, it might be set to warn you when you are exceeding the speed limit. or something like that. Spend a bit of time going through the menus to familiarise yourself. Different models are different.
You can also download a manual. For example I have a One XL and it is on
http://download.tomtom.com/open/manuals/onexl/manual.htm but if you navigate around their website I expect they will have one for yours.