(and causing the cursor to "disappear" too.) I was going to swap back to the old devices, to help detect the suspect item, when I learned about the Keyboard Viewer, and then saw that the deletex> key (and later also obvserved the F13 key too) was stuck. so I certainly don't think it's the computer (no recent OS upgrades, nor RAM etc). I have an old clear plastic Apple keyboard and mouse from my eMac that I can use to rule out one or the other, but I also know the problem did not exist until I changed them both for the new wired keyboard and Trackpad.
But the reason I was dismissing it was that after keeping Keyboard Viewer open, I could see when the BEEPs were occurring, either the F13 or deletex> keys were showing as active (despite not being pressed), and the BEEPs would stop when I pressed a key on the keyboard - not when doing anything to the Trackpad. Furthermore, Apple says that keyboard-related repairs should be prioritized, with stores. RE the mouse - I'm actually using a Magic Trackpad. Apple explains that it has shipped additional service parts to stores to support on-site keyboard repairs. for the 2 at fault to be essentially next to each other too, made me wonder. If not actual liquid, I was assuming "something" had gotten under the keys. G'day seventy one - thanks for the reply. I've taken the cap off both, and sprayed some compressed air around. They are however not actually working when you want to press them. I learned about the Keyboard Viewer App (via the Keyboard control panel), and discovered that the F13 key, and the "delete x>" key were both randomly lighting up and staying light up (ie as if being pressed") all on their own. This again would be resolved after pressing a key on the keyboard. Then a few times, the computer made repeated system-beeps - ie beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep beep! It did not appear to be a mouse issue, and the cursor would return to normal after pressing a key on the keyboard. The cursor would disappear, but re-appear if you moved the mouse, then disappear again.
I recently purchased a 2nd hand "full" wired keyboard (A1243), and after a short while noticed that it was causing issues with my Early 2009 iMac.