People will undoubtedly mention Ravel's Bolero as a classical piece with
a continuous drumbeat.  How about "Mars" from Holst's Planets, in which I
think the drumbeat is continuous, or if not, it drops out for only a very
short time.  There're also wonderful rhythmic idees fixes in the second
movement of Sibelius's Fifth and in the finale to Holst's St.  Paul Suite
(and 2nd Suite for Military Band).

Steve Schwartz