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Update:
As I mentioned earlier, the engine tends to become noisy (sounds like a knocking sound) between 3K-4K RPM range.

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plus a number of people asking about the bike in signals and petrol pumps. I didn't expect it to be a head turner, but surprisingly it is may be because of less number of GS on road.

20W40 is recommended in the manual.

The available choices in motul 7100 are 10W40 or 20W50.

Please suggest.

Update:
As I mentioned earlier, the engine tends to become noisy (sounds like a knocking sound) between 3K-4K RPM range.

I would try capturing the sound and post it this weekend. It actually sounds similar to the knocking sound we usually hear when we ride on a higher gear in slower speeds and the engine is not able to pull the bike. It is evident between 3K-4K rpm and then gradually fades away.
I had complained this issue in 3 different service centres in Chennai - Avenue Suzuki (worst customer experience as far as bike service and customer service is concerned), Pramaan Suzuki and ABT Suzuki. All the svc centres say that the sound is expected in GS and it is not an abnormal sound. My trustworthy mechanic, though has not worked on a GS before, with his many years of experience says that the sound seems to be due to high compression. He says that it'll not affect the engine life but the sound may be there forever if it's not rectified.

I already faced the hazzle of going back and forth between difference svc centres reporting the problem. I did not want to spoil my peace of mind by trying to ask the svc guys sort out the issue which they say is normal. So, as of now, I am trying to ignore the sound and enjoy the ride.

And yes, the service technicians have their own imaginations. The svc technician in Avenue Suzuki told me that the sound is nothing but the silencer beat. He compared Unicorn and Apache and said that the newer versions of GS has an open mouth silencer like Apache and thus it is louder . They consider the customers to be so naive. I just left the place since I thought there was no point in explaining him that it's not the sound from the silencer but from the engine .

If you're talking about vibration feeling you get around 3.5K RPM then we(earlier owners) called it `resonance`. We used a rubber bush to minimise the effect. I bought one from Bajaj spares shop for ₹ 5/-

gmagesh has a link in his signature for the procedure.
Edit: here the page - http://www.xbhp.com/talkies/815716-post16175.html

In which gear?.

The GS I test rode in Chennai, Nandanam showroom had the same noise and even I had mentioned this when I wrote down my riding impressions in this thread.
I asked the sales guy about this and his answer was - "the bike's PDI is not done, and will be taken care during PDI"

And, this was the only thing that annoyed me during test ride.

Update:
As I mentioned earlier, the engine tends to become noisy (sounds like a knocking sound) between 3K-4K RPM range. The service guys say that this sound is expected from GS because of the engine balancer. I also rode another GS which also had this weird knocking sound. The bike is pretty smooth except for this sound. Anybody else facing such abnormal sound? I heard that even bikes like FZ has an engine balancer. So, I doubt if this is the real cause for the sound. The service supervisor says that GS has a double balancer. Is it true?

Anyways, I do not regret one bit on buying a GS. Yes, it isn't sporty as an Apache or eye-catchy as an FZ, but it does it's job absolutely perfect with exceptional ride quality and very good mileage, plus a number of people asking about the bike in signals and petrol pumps. I didn't expect it to be a head turner, but surprisingly it is may be because of less number of GS on road.

I did not refer to the vibration due to resonance. I agree it is quite normal and is not specific to GS. I was talking about a knocking sound from the engine.
I tried taking a video of the sound but the video doesn't do any justice at all. One has to hear it practically while riding.

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Irrespective of the gear, the noise just crops up betwen 3-4K RPM and then fades out gradually.

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So, does this mean the noise is the nature of GS engines? I don't want to make an issue out of this problem again with the service guys, unless it is actually a problem really. If GS bikes are meant to have that sound (GS owners can enlighten me on this), then I am perfectly fine with that. I just wanted to make sure nothing's wrong with my new bike.

And regarding the response from the sales guy you met.. The service guys are themselves not very confident in some svc centres, how can we expect the sales guy to be aware of such engine sounds?

today i completed 1000 kms & took my bike for servicing. Everything was good before it. But after servicing vibration noise is coming from front faring of bike. I was there at the time of servicing and watched the mechanic while he was washing bike. Washing was with very high pressure water jet & at the time of washing he aimed water jet on headlamp constantly , when i asked not do it then he reduced water jet speed. But i think vibrations are because of it only. Does someone know how to rectify it now ?

But after servicing vibration noise is coming from front faring of bike. Washing was with very high pressure water jet ... i think vibrations are because of it only.

Even mine had some rattling noise from the headlamp cowl... I guess they would insert a piece of cardboard/sponge to hold the headlight assembly tight.

I was talking about a knocking sound from the engine. I tried taking a video of the sound but the video doesn't do any justice at all. One has to hear it practically while riding.

Sounds way too different, more like a Bajaj autorickshaw/UV's diesel engine in the recording. Actual engine sound/exhaust note is fantastic!

The instrument panel is fitted on rubber bushes and is supposed to vibrate a little. Washing with high pressure water or tap water does not cause vibration.
If the gap between the console and ignition switch or headlight assembly is less than required, get it checked and adjusted. Or try to enjoy that vibration if it is not an annoyance instead of inserting something for tightness.

Just try to ride the bike as it is as your bike’s engine and the sound from is perfectly OK. If not convinced, record another GS’s engine sound and compare. The sound from the engine you hear while riding, riding as a pillion rider, as a pedestrian or from a recorded source is totally different.
Use the free time you spend around the engine to listen to its sound to wash and polish the bike, to lubricate the chain and to adjust its slackness, to check the tyre inflation etc.
I hope I am correct.