Bluetooth LE works at 2.4GHz, the same frequency used for WiFi and microwave ovens! There are two main types of Bluetooth LE transmission: advertising where the device only sends out data and connection-orientated using Generic Attributes (GATT) .
Bluetooth LE uses frequency hopping for increased reliability when there might be noise on a particular channel. When advertising, it uses channels 37, 37 and 39. If subsequently connected, it uses other channels:
Frequency Channels from RF Wireless-World
When only advertising, the transmission is very short of the order of 1 or 2 milliseconds. In between advertising, there is no transmission which is one of the main reasons why Bluetooth LE has very low power use:
Read about Choosing an Advertising Interval