A man fired a catapult at a lorry driving along the A40 smashing one of its windows.
The lorry, driven by a Cinderford man, was passing by the hand car wash centre in Birdwood when the passenger side window of his cab shattered.
The 59-year-old driver said he saw a man standing by a blue Ford Transit van holding a catapult.
The offender is described as a white man wearing a red vest who had dark hair.
The incident happened between 1pm and 1.15pm on Friday.
Anyone who witnessed the incident or the man or van involved in the incident is urged to contact police on 0845 090 1234, or Crimestoppers anonymously on 0800 555 111, quoting incident 248 of June 3.