Is there anything that can be done to completely stop an annoying, "loud,
thud noise" that a duct is making? Repeatedly, about every 10 seconds, there
is only one duct in the basement that must be expanding/contracting and then
makes a loud thud sound.

The basement is unfinished and it's been difficult to determine exactly
which duct is making this noise. The noise is occuring only late at night
(and we haven't got a clue why it doesn't happen during the daytime) and the
loud noise is quite irritating (there are no ghosts that we know of).

We've checked to make sure that all the basement ducts are fully opened and
have also opened all the registers located in the ducts ... at first, these
adjustments seemed to lengthen the time between the "loud, thud noise" but
it has not entirely stopped the noise.

Closing the register in one of the bedrooms also seemed to helped, but
leaving this register closed is not a viable long term option. We suspected
this was helping because it is trapping heat for a longer time in the duct,
but we don't know for certain.

The home and furnace are 18 years old.

Thanks.