...Ahh! I'm so sorry I didn't notice....it would
be tinnitum if you were using the passive form of the verb (in passive perfect forms, verb endings follow noun genders....so "puella tinnita est", "puellae tinnitae sunt", "corpus tinnitum est", like those make sense or something). But that's the passive form..."Tintinnabulum tinnitum est" means "the bell has been
rung." Honestly, leave it up to me to botch answering things like that, should have left it up to benissimus or something.
The bell has rung = tintinnabulum tinnivit. (3rd p.p. is tinnivit, and since the bell is 3rd person singular, you add the ending -it.)
Sorry for all that trouble!!!
edit: puellae tinnitae sunt
edit 2: saying I edited it.
edit 3: corpus is neuter. will someone please just delete this thread?