I have a 2006 Nissan Sentra 1.8l. I had a bad tensioner and old belts. I replaced both belts with the Dayco polyribbed belts and put on a new tensioner pulley. All went well and engine noise is gone; however, when starting the engine I get a squeak in one of the belts. I’ve tightened each belt a bit more several times. I still get one short squeak every time I start the car. I loosened one belt so I could spin the waterpump by hand. It felt and sounded fine. Do I have a problem here or are these Dayco belts prone to this? The old belts were gator belts. I suppose I could swap them back and see if squeak disappears.
Try a touch of “Belt Dressing” Spray… Another item I can think of that fools people into thinking its a loose belt is the Alternator bearings…you can try to lube if suspected.
The item that stumps them all is the Harmonic damper on the crank…Most times they are cushioned in rubber… Think two circles…one small…one large…the small circle is in the center…and the large the outer surface where the belt is driven by the crank…there is usually rubber in between these circles…to help dampen the crank vibes… This rubber hardens and shrinks over time and upon startup it allows the inner circle of steel to slip…and squeak… Fools em every time.
Start with the easy suspects first…then move on till you wind up at the Harmonic Damper. Belt Dressing, Alternator…Hell even the starter can fool you sometimes…but each has its way of telling you what it is. Sometimes the garden hose is useful in figuring it out…wet the suspect and if it shuts up…or gets worse…you found it. LOL
Blackbird
I would loosen the belt so you can spin every pulley by hand. Any that feel rough will point to where the problem lies.
You don’t need a garden hose just a spray bottle with plain water will dampen things enough.
Yosemite
+1 to Yosemite’s post. When checking them, also try wobbing them laterally and axially (sideways and in & out) checking for play.
Can I assume that you changed the entire tensioner and not just the pulley? The Sentra tensioner uses a coiled flat spring, and if the spring is getting tired it won’t keep the belt at the proper tension. It could be slipping on startup and still be quiet while running. It takes more friction to get the pulley started than it does to keep it running.
Why is there not enough water in the garden hose ? Or perhaps there is no garden… nor hose…
Huh???
I was messing with Yosemite…lol
Got it.
I get lost easily. More so in more recent years. 
Your alternator belt is not tight enough. It can be difficult to judge belt tension when the pulleys are close together, reach down to the section of belt between the bottom two pulleys to check tension.
A word of caution, if you spray V-belt dressing on a serpentine belt it will be so noisy you will have to replace the belts.
A word of caution, if you spray V-belt dressing on a serpentine belt it will be so noisy you will have to replace the belts.
And clean all the belt dressing off the pulleys.
@Honda Blackbird; Don’t mess with me!!! I was a good shot with my BB gun as a kid…knockin off those Red wing Blackbirds. But then, with only one eye, I may just shoot myself in the foot. 
I thought that instead if getting a free shower, (been there, done that)…and it’s not even the first of the month…a spray bottle may work better. I also did think of the fact that this poster may live in an apartment and no hose is available.
Plus, I’ve found that a little water from a spray bottle will allow you to concentrate on just one small area and not soak everything. It seems that when you soak everything the squeak goes away too long of a period of time. Then you have to let it all dry again so the squeak will return and you can dampen another pulley.
I have found it easier to lightly spray one pulley at a time. The spray will dry within a minute or two and you can try another pulley.
Soaking with a hose will take the squeak away, but a 10 mile drive and it’s back.
Yosemite
You are correct and I agree with you… I was messin round
Thank you one and all for your thoughts and suggestions. I shall get to work on it and see what I find. Many thanks.
Belt dressing attracts dust and dirt and causes premature wear.
One more guess for the mix, replacing a pulley incorrectly or with the wrong part could cause the set of pulleys which comprise the belt routing to not all be aligned in the same plane. Try to position yourself so you can see them edge-on, maybe you’ll notice the one you recently installed is sticking out or in a bit too far. And double check the new pulley has the correct number of grooves.
TSM’s question is a good one too, whether you replaced the tensioner pulley or the entire tensioner.
Knfenimore is right about belt dressing…it sorta turns into a glue after a few min…first its slippery…then its sticky. Weird Wild Wacky Stuff I tell ya But true
Nissan Published A 5 Page Technical Service Bulletin For Their Technicians To Assist Them In Diagnosing Belt Noise. It Includes A Flow Chart, Color Drawings, Items Specific To Certain Engines, And Solutions.
It wouldn’t hurt to search, download, and take a look at the Nissan TSB, #NTB10-002 (Jan 12,2010).
Oh, and does this vehicle have a “one-way clutch” pulley (mentioned by Nissan) on its alternator?
CSA
sounds like ;
’’ 27, 8 x 10 color glossy photographs with circles and arrows and a paprgraph on the back of each one explainin’ what each one was. ‘’
( pop quiz , anyone remember that ? )
Arlo Guthrie, Alice’s Restaurant, Of Course.
CSA