2005 Dodge Grand Caravan SXT with 120K on the clock, has had regular maintenance. Engine makes an unpleasant "whoomp whoomp whoomp" noise at idle which seems to go away under load. Vehicle has been taken to Pep Boys and they ran numerous diagnostic tests to determine if there were any problems and they said vehicle passed and runs well. They tested wiring and said there was no mis-fire due to that. Vehicle has been on several recent 1200 mile trips with no problems. Vehicle has passed exhaust emissions tests and has no smoke or other problems. Can anyone let me know if they have experienced this and if there is a solution?

Thank you!