Shmoop will totally admit it: drums in The Lord of the Rings creep us out. There are the drums in the deep of Moria in The Fellowship of the Ring, and the hosts of Mordor arrive at the fields of Pelennor accompanied by the sound of siege drums. Shiver.
It's a good thing there are all these horns around, then. To fight back against the rolling beat of the drum, the good guys use horns. Aragorn blasts a silver horn at the Stone of Erech to call the Sleepless Dead, Boromir's horn is what alerts Faramir and Denethor to his death, and as Théoden and the Riders of Rohan approach the Fields of Pelennor, they announce themselves with horns:
With that [Théoden] seized a great horn from Guthláf his banner-bearer, and he blew such a blast upon it that it burst asunder. And straightaway all the horns in the host were lifted up in music, and the blowing of the horns of Rohan in that hour was like a storm upon the plain and a thunder in the mountains. (5.5.65)
The contrast that Tolkien builds between the troops of good and evil is so strong that the two sides don't even use the same musical instruments to announce their coming. We have no idea why Tolkien finds the drums so scary. Perhaps because the steady, relentless beat suggests the unstoppable approach of these evil guys? Whatever the reason, Tolkien's imagery isn't only based on what the reader can see. He also asks the reader to hear the approach of the orcs of Mordor, which gives us a more three-dimensional and rounded sense of the world of Middle-earth that he's building. And it also gives us a bit of a chill.