What's up with the 1966 cent?

I did a copper cent sort this week and when I do I habitually bounce the coins on a hard table top to hear the copper ring. I have come across a third 1966 cent that makes a thud sound when it hits the table. I mean a thud. Not a ring, nothing. Does not sound like a zincoln thud either. Sounds like like one of those magician coins that has been machined out and fitted with another coin's obverse actually. Could the mint have had a bad run of cents in 1966 where air bubbles or an internal fracture was present in the planchets? The first 1966 cent I found like this was a few years ago and I took it to a coin dealer at a show and he brushed it off saying he could hear the coin make a distinctive sound. Barbara Streisand. Now, these 1966 anomalies can not be rare if I found three so easily without a heck of a lot of sorting. The individual weights of them are a little under and a little over the standard 3.11 gram weight. Oxidation and crud may cause that, so weighing is inconclusive. It is only the year 1966 also. Thousands of other cents from pre-82 I have bounced and they all ring nice when tested. Where is the Coin Mystery Search Team?