Sounds like you might be hearing the EBPV. It is a butterfly type valve just after the turbo. It is supposed to aid in warmup by providing a load for the engine. Has it recently gotten colder in your area? If my memory is right, the EBPV is activated when EOT and ambient air both drop below a certain temp. If this is your first winter with a non-stock exhaust it can certainly surprise you when you first hear it. Some people unplug it so it doesn't activate and others completely gut it from the truck. It can be off and on as your driving as sometime it 'blows' open. It really sounds like the EBPV and I don't think you have anything to worry unless it never goes away. With the colder weather it can take nearly 15+ minutes for me to get EOT up warm enough on my '95 that it quits.
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Thomas H.
'97 F-250 CCSB 7.3L Auto, Gauges, Modded H2E, 285/200% sticks, 7.3L IC, Stealth Dual HPOP, Pheonix chip
'03 F-250 CCSB 6.0L Auto, Gryphon CTS, FICM tuning by PHP, Powermax
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