  We consider a conservative system consisting of an elastic plate interacting
with a gas filling a semi-infinite tube. The plate is placed on the bottom of
the tube. The dynamics of the gas velocity potential is governed by the linear
wave equation. The plate displacement satisfies the linear Kirchhoff equation.
We show that this system possesses an infinite number of periodic solutions
with the frequencies tending to infinity. This means that the well-known
property of decaying of local wave energy in tube domains does not hold for the
system considered.
