I think chipmunks get a bad rap on living in homes. If you have a seasonally-used chimney, it's a good idea to cap it. Chipmunks will chew their way
out of homes.
They are not particularly
good chewers, so they may have entered thorough a "large
enough" existing hole , or one previously chewed by red squirrels (who
are good chewers).
Although they are decent climbers, chipmunks prefer to live in burrows in the ground. I unintentionally provided ideal quarters for them by dumping a small pile of "rip-rap" rocks to support a driveway's edges.
If it's a "family-of-four", then a one-way door may prevent feeding of youngsters. (Male chipmunks don't assist in raising the young).
By the end of June, they should be out of there. Then seal the entry with "hardware cloth" -- a metal mesh -- available at...um...hardware stores.
.