We consider the equation {Delta(2)u+c Delta u=lambda f(x,u), x is an element of Omega, u=Delta u=0, x is an element of partial derivative Omega, where Delta(2) denotes the biharmonic operator, c is a given constant, Omega is a bounded domain in R-n(n >= 1) with smooth boundary partial derivative Omega, and lambda>0 is a parameter. The nonlinearity f exhibits an oscillatory behavior. We establish the existence of multiple positive solutions, multiple negative solutions, and multiple sign-changing solutions, depending on lambda.