Yes, in 2Kings 5:18 Rimmon is referring to a Syrian god. There isn’t much information about this false god, either inside or outside the Bible. The International Standard Bible Encyclopaedia has the following entry:
A Syrian god. Naaman the Syrian leper after being cured is troubled over the fact that he will still have to bow down in the house of the Syrian god, Rimmon, when his master goes into the house to worship leaning on his hand (2 Kings 5:18). Elisha answers him ambiguously: “Go in peace.” Judging from Naaman’s position and this incident, Rimmon must have been one of the leading gods of the Syrians worshipped in Damascus. He has been identified with Rammanu, the Assyrian god of wind, rain and storm. The name appears in the Syrian personal names HADADRIMMON and TABRIMMON (which see) and its meaning is dubious (ramamu, “to thunder” (?))1
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Notes
1. Nathan Isaacs, entry ‘Rimmon (2)’ in International Standard Bible Encyclopaedia (Wm. B. Eerdmans Publishing Co., 1939). Online: http://www.internationalstandardbible.com/R/rimmon-%282%29.html (accessed 23/05/10)