300 6-cyclinder- Intermitant whistling noise, loud whistling noise when driving under a pull. From 40-60 MPH Items checked - vacuum at idle is good - 18 1/2 - 19 inches - checked air pump - seems to be working when vacuum is applied to diverter valve and get air flow going to manifold check valve. pulled hose off check valve and can feel vacuum and exhaust pressure not sure which due to pulsation. Hose was not burnt.