I have a 1995 3500 2WD with 95000 mi. I've owned it for 1 year and have upgraded cooling system, exhaust system, relocated PMD, new lift pump, new OPS, turbomaster and upgraded eprom, S&B air filter and two weeks ago replaced old fuel injectors with new Bosch standard output injectors. I had a local mechanic familiar with 6.5's replace the injectors. I drove the truck last week and could hear a strange but faint noise that I initially thought was something rattling in the glovebox, dash or floorboard. I could only hear the sound when traveling in 5th gear. I sat in the passenger seat while my fiance drove and it sounded as if the sound was coming from under the hood near the turbo. After stopping and raising the hood, I thought I could hear a slight "knocking" sound at idle on the turbo side of the engine. The sound is audible only when in 5th gear or when at idle and the hood raised, hot or cold, towing or not towing. I've read previous posts and read that air in the injector lines, bad injector, etc, could be source of noise. How do I determine what the source is? What should I look for or check before taking it back to the shop? Are there other signs or symptons to look for that would indicate what the issue is?

sounds like the injectors knocking

So is there something I can do before taking it back to the shop? Would there be any loss of power or performance issues or warning lights? I called the mechanic and he was unsure why the noise would be heard now so that's why I'm asking on here. Is this a case of a bad injector or something out of tune or what?

mebby exhaust piping hitting the frame somewhere,
Check the damper pulley too.

I've checked for loose parts, heat shields, etc. Noise started right after injectors replaced and sounds like a "knocking" on the turbo side.

Turbo heat shield not fasten tight enough? This is the one that goes between the turbo and the block.

It didn't feel loose. I'm going to double check everything but was hoping for some advise if the issue is the injectors. What some other signs or symtoms might be if this is the case or if is is a case of improper installation or adjustment.

Maybe dirt in the injector line, from when they were off and the job was being done, is now partly plugging one injector. It doesn't look to me like it would take much of that to cause a problem. Also, I seem to recall someone saying that it takes a little time for the injectors to break in. Maybe someone will chime in on that. I know that mine blew a little white smoke for the first 20 miles or so but are fine now.

It's been driven at least 400 miles since replacement.

Try using a stethoscope (I use an aluminum arrow shaft with a vinyl tube attached) and listen to each injector. Also, touch the shaft to the injector line itself and see if that makes it more obvious as to which one is making the noise. You may just have a bad injector or one that has some crud in it. I should think they would be pretty well seated after 400 miles.

could be turbo itself, check for play in the wheel with the engine off of course lol, and how loud is it at idle the knocking? You could do a power balance test, when the engine is running crack open a injector line then close it, everytime you crack one open the engine will lose rpm bc of that cylinder, if all injectors are firing evenly to say then the rpm and sound drop would be almost exactly identical.

Thanks for the input. I'm going to take it back to the mechanic tomorrow and have him check some of this.

injectors can fail without notice,even new ones.
With 400mls on it is enough time passed to exclude dirt introduction failure.

I went through when I changed my injectors. Mine turned out to be an injector line touch the valve cover. Make sure the lines are not touch each other or anything else. BTW I found my problem just by pushing and pulling slightly on the injector lines. It's worth a shot!

All the maneuvering under there could have shifted things during the exchange and now an injector line could be making contact with the aluminum portion of heater hose which will manifest itself through the glove box.
Also, I've been told it can take a while for the new injector's internals to seat so it could be one of them fussing a little till that's done.

On 2 different vehicles i have had to bleed the injectors while having the engine running @ 1500 rpm. Both times the engines sounded like they were knocking right after i replaced the injectors.