loud engine noise after 5 to 10 mile haul up 6000'alt

[letsgo]

On a number of occasions after a 5 to 10 mile haul climbing to 6000' and 8000' towing in towhaul mode, an extremely loud noise would come from the engine area just before going over the crest of the hill/mountain, speed was 55mph @ 2800rpm to 3200rpm the electronics were in compleate control, not sure whether it was the engine fan,the noise would instantly disappear as the truck and trailer would reach the top and upshift to a higher gear, neither the rpm or speed changed when this noise occured the engine temperature was always constant going up or down hills the tranny temperature would fluctuate a 1/16th, I looked at the tranny oil, it looked OK and smelt fine, at one point I had ONSTAR do a diagnostic while the noise was taking place, no fault codes detected.

any input would be welcomed,

[ 05-17-2004, 10:44 AM: Message edited by: letsgo ]

[Rockin]

Probably your cooling fan engaging. I can't explain why it engages when the coolant temp seems to stay constant.

[ore.diesel]

the first time the fan kicked on in my 6.5 I thought it was ready to blow. haven't had it kick on with my dmax yet but sure that it will this summer.

[Nevada Hauler]

Does it happen at 2600 rpm and above? If so, it just might be the pilot injection cutting out.
Pilot injection is a small early shot of fuel into the cylinder which assists in more even cylinder pressure buildup( one of the reasons the Duramax is so quiet for a diesel)

[letsgo]

Nevada Hauler: yes the rpm was always in excess of 2600rpm, the truck would be running at the same speed and rpm for 5 to 10 minuites no down shift took place, again engine temperature was always constant (before my engine fan comes on the engine and tranny temp. have to really shoot up past the half way point on both gauges) the noise always took place before the crest of the mountain. I cant tell the difference between engine noises when the pilot injectores are on or off.