climate control

1987 420 SEL.
Climate control is set which requires a mixture of cool and hot air. Temperature should remain constant.
As the climate control regulates itself heat is sent into the cabin for an extended period. Sometimes a "clap" is heard as if a door is suddenly closed, prior to the heat abating. Sometimes after the car is turned off and then restarted while hot, hot air comes into the cabin for awhile before the temperature finally reaches the set level.
Any suggestions?

[This message has been edited by hewv7 (edited 05-11-99).]