You ask "if there is a more mundane explanation" and note that you're "not looking for a diagnosis." As you'd expect of course, it seems sensible to conclude that indeed no definitive judgement can be made about the cause of your experiences without some sort of professional assessment. And there are methods available to attempt to make such a determination.1 My cursory research indicates that these are in the process of development, that there is nothing "official," but that some are described as "reliable." Whether there are any applicable to sight-to-taste sensations, I can't say.
As you note, sight-to-taste synesthesia typically involves things like letters, words, numbers, colors, or shapes. I did find a few examples of perceptions similar to yours. One relates to a man who experienced synesthetic taste sensations of "synthetic inedibles (wax candles, for example)."2 Another is a reference to the idea that some synesthetes "may connect the sense of sight to taste so that every time they see a sailboat they will taste a donut'."3 Finally, there is another mere supposition that "llamas [may] look like sweet and sour sauce."4 These examples seem to confirm an observation that "[t]here are a lot of synesthete stories around the Internet, but many of them are lost within blog comments, outdated forums or hard-to-read Tumblr threads."5
"A more mundane explanation," in this case related to more common forms of "blended senses," is found in the theory that "people who have Synesthesia just retain color memories from childhood. An example would be colorful alphabetical or numerical magnets that the child had played with. This theory, however, does not explain all aspects of the condition."6 It seems impossible, based on the information available, to even speculate about the likelihood that your sensations result from childhood experiences, although you do note that you may have " tasted [cooper and carpet] when [you were] a child."
You may be interested in a recent study of so-called " learned synesthesia," examining "whether traits typically regarded as markers of synesthesia can be acquired by simply reading in color."7
Is there any history of synesthesia in your family? "It is believed that one gene which is responsible for causing Synesthesia gets passed from one generation to the next by the X chromosome, as a Dominant trait (Smilek, 2005). This could explain why Synesthesia runs in families."8
Finally, you may be familiar with the work of Richard E. Cytowic, a medical doctor and an Associate Professor of Neurology at George Washington University, and a leading authority on synesthesia. He is the author of The Man Who Tasted Shapes, and was featured a few years ago on the American television program "60 Minutes." He may be interested in your experiences. You might want to contact him.
1 "There are several screening techniques that are used in diagnosing and confirming Synesthesia. … These are called pop-out, segregation, cross-modality imagery, and the Stroop test." — from Synesthesia: The Medical Condition of Associating Letters and Numbers With Certain Colors (2007), by Jennifer Rossman, citing Crane, Carol A. (2006). Synesthesia. A neuropsychological and familial study of developmental synesthesia. Dissertation Abstracts International, 66(8-B), 4477,
2 A Summary of Current Ideas on Synesthesia (2007?), by Megan Davis, for an undergraduate seminar in Cognitive Psychology at Goucher College.
3 A FAQ about synesthesia on a blog published on wikispaces.com.
4 A document about synesthesia published on everything2.com
5 Maureen Seaberg’s Synesthesia Story (Video), a blog post on blendedsenses.com
6 Rossman, citing Ramachandran, Vilayanur S. & Hubbard, Edward M. (2005). Hearing colors, tasting shapes. Scientific American Mind,16, 16-23.
7 Pseudo-Synesthesia through Reading Books with Colored Letters (2012), by Olympia Colizoli, Jaap M. J. Murre, and Romke Rouw.
8 Rossman, citing Smilek, Daniel, & Dixon, Mike J. (2005) Synesthesia: Discordant male monozygotic twins. Neurocase,11(5), 363-370.