Sorry if this is a repeat,but after doing a few searches,I can't find my problem.On my '98.5 24 V,5 spd,4x4 3500,When empty,accelerating up very steep grades, (10%-15%)3rd gear, at about 2300---3000 rpm I sometimes get a quick whumpa,whumpa,whumpa sound from what seems to be the center of the engine compartment.It has happened in 4th,too at about the same rpm,but usually only when towing.There's no loss of power,oil press,or temp.rise.It goes away quickly when rpm is lowered or raised,and it doesn't do it all the time.It's done it from since about 20,000 miles( now has 60,000). It's now off Cummins warranty,but It would never do it with dealer mechanic in the truck,of course.It kinda sounds like someone quickly whacking something with a rubber hose.I can't find anything loose or out of place.Has anyone else had this problem? Please help,I hope it's not serious,truck runs awesome,good power and fuel economy,but this noise is worrisome. Thanks,all!

 

I believe there were several threads on this subject. If I rember correctly there was an updated thermostat, to correct this issue. Try doing a search on thermostat.<br>

 

Another thing you could check is the oval plastic hose from the air cleaner box to inner fender.Mine hadn't been tucked into the fender and was trying to fold up on itself while towing in the mountains. After attaching it to the inner fender no more noise!<br><br> George

 

Thanks for the replies.I did a search on thermostat,and thermostat noise,and I couldn't seem to find anything like my problem.I'll check out the oval air duct hose thing today and see if that's the problem.

 

This is a long shot, a wild guess, but could it be possible that the fan clutch is cutting in and out rapidly, causing the fan to start and stop?
When you do finally resolve the problem please let us all know the cure.

 

georgej,I did find that the plastic oval airduct piece was separated from the air box.I popped that back into place,and I will let you all know if that solves the noise problem.Thanks for your thoughts!