99 p71 sounds like water sloshing around under dash of passenger side.

After all the rain that happened last week I now hear some water 'sloshing' around in or under the dash of front passenger side. With windows up and radio off I hear it when car starts moving and while driving. NO WATER on mat or under it.
It is a 1999 p71 135k miles. Any tips / help is welcome. Thanks.