hi bud, it is quite common for the older mounts to come away inside due to the heat of the motor, some had little alloy collor to deflect the heat, some were fitted with the slightly larger mounts with different angled stantions and a different liquid inner, they look like the trd stlye mount.
i had an issue with mine and it took my best part of two years to find, only found it by taking the engine out. the passenger side one was knocking when i used the brake, or reverse then used the brake, it felt like a clunk but would only do it once! and to me it felt like the drivers side one so i replaced that and it still happend.... now i know why.