“Summoning sickness” does a very specific set of things: it prevents the creature from attacking, and it prevents the creature from activating abilities with the tap or untap symbol until the creature has started your turn under your control. As long as the ability does not use the tap or untap symbol, the ability can be used right away. And, of course, only creatures are affected by “summoning sickness,” so the Drum can activate its ability immediately.
While the Drum’s ability does use the tap symbol, that’s referring only to the Drum itself, not to the creature that it taps as well. So you can tap a freshly cast creature to activate the Drum’s ability. This probably explains the power of the card in Modern, and why it’s a strong consideration for a place in an inspired deck as a quick way to “turn on” your inspired creatures.
Today’s Rules Tip written by Nathan Long