earworm

Short definition from Wiktionary

Noun

1. 1
   A tune that keeps replaying in one's head or that one keeps thinking about, especially if unwanted.
2. 2
   Ellipsis of corn earworm (“larva of the moths Helicoverpa zea (syn. Heliothis zea) and Helicoverpa armigera, which are agricultural pests”).
3. 3
   An earwig (“insect of the order Dermaptera”).