Welcome to the forum. The 347th was an Engineer General Service Regiment.
The lineage of the 18th Engineer Brigade can be traced back to 29 July 1921, when its predecessor unit, the 347th Engineers (General Service), was first constituted as an Organized Reserves unit. The 347th Engineers would not be activated for almost twenty years, until the United States began its military buildup following its entrance into World War II. The unit was ordered into active military service on 6 May 1942 at Camp Claiborne, Louisiana.
On 1 August 1942, the unit was reorganized and re-designated as the 347th Engineer General Services Regiment. The Regiment deployed to England in February 1944. It entered combat in France on 29 June 1944 and participated in the Normandy, Northern France, Rhineland and Central Europe campaigns of World War II, earning its first Meritorious Unit Commendation. After V-E Day, the Regiment remained on occupation duty in Germany until it was inactivated on 1 June 1946.
Do you have any documents that show your grandfather was at the D-Day invasion on June 6 or 7? if so he may have been in a different unit & joined the 347th at a later date.