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2009 Forester XT Auto
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218 Posts
Discussion Starter #1
Perhaps this has happened before to someone else but I could find the right keywords in search to define my symptoms.

All fluids were changed at 34k miles. I am now right at 62K miles.

It is hard to explain and only seems to happen after driving for a bit. It seems to occur between 20-40mph when the engine is between 2-3k RPM. Not hard driving just lightly through level city streets with a little acceleration.

I get this loud humming that transmits through the entire car until I get close to 40mph. The faster you go the more the pitch heightens eventually to the point of not hearing it anymore. So far I can detect no noise on the highway outside of what I normally hear.

My first thought is a fluid change again, perhaps something better than regular gear oil. This I could do myself as last time I had a local subie specialist do it for me.

The car leaks nothing and I see no seepage as well. I have no driveline mods as well but I'm also thinking maybe it's my mounts. They could have just worn to point of transmitting normal NVH instead of isolating it.

All help appreciated.
 

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04 Forester X, MT
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1,039 Posts
It may be rear diff whine. Go with a good quality synthetic 75w90 LS oil in the rear diff. Mobil 1 and Castrol are commonly available around here about $10/qt. I even added half a tube of K&W LS additive in my rear diff and it is relatively quiet compared to 80w90 dino oil before.

Do NOT add any gear oil with LS additive in the transmission because it is not compatible with the synchros. I didn't want to pay $20+/qt for some of the high-end synthetic stuff but had trouble finding synthetic oil compatible with MTs. I am now using Lubro-Moly 75w90 from Napa and am pretty happy with it so far at maybe $12/L. Definitely better than cheap dino oil I had in there previously.

edit:
if you go with new mounts and get "performance" bushings and the like, you will likely get some better "feel" but even more NVH in the cabin.
 

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2009 Forester XT Auto
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218 Posts
Discussion Starter #3
I think it might be rear diff whine, it does come from the back.

He offered synthetic fluid last time but for the price I said regular would be fine. At that time I had no garage so I couldn't do it myself. I will give synthetic a try since it technically is due again anyway.

Have you ever tried GM Synchromesh? I recommended a friend with a hard shifting Accord try it and he said it did wonders. Considering the fact that Subies have a tran & diff in one I was worried it wouldn't be compatible.
 

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2010 Forester 2.5X
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1,692 Posts
I believe the GM gear oil has friction modifier in it. I say "no" for manual transmissions.
 

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2002 Forester S
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270 Posts
I think it might be rear diff whine, it does come from the back.

He offered synthetic fluid last time but for the price I said regular would be fine. At that time I had no garage so I couldn't do it myself. I will give synthetic a try since it technically is due again anyway.

Have you ever tried GM Synchromesh? I recommended a friend with a hard shifting Accord try it and he said it did wonders. Considering the fact that Subies have a tran & diff in one I was worried it wouldn't be compatible.
Are you saying put GM Synchromesh, which is an MT oil similar to ATF but I believe a bit thicker, into a Subie 4EAT?
 

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Registered
2009 Forester XT Auto
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218 Posts
Discussion Starter #6
Are you saying put GM Synchromesh, which is an MT oil similar to ATF but I believe a bit thicker, into a Subie 4EAT?
No a 5MT which from what I just read is a no-no.
 
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