Helping a friend who bought a beautifully restored Yamaha RT3 360. Stock pipe is still on bike and has baffle intact, but man the bike is super loud, as if no baffle at all. It is hard to tell looking at exhaust pipe diagram what might be missing. He wants it to sound like a stock bike. Wonder if someone may have gutted internals? Any ideas? The exhaust pipe outlet PN 291-14752-30-00 has no fiberglass silencing. Any ideas?

Verify the bike, that part number is for a '73 not a '74. Very different exhaust. Back in the day I had a '71 DT1E 250 that the internal baffle welds broke loose and was obnoxiously loud even for a 15 year old teenager. Dealer replaced the entire exhaust pipe under warranty.
Does the pipe rattle when you shake it?
I have NOS and early take off exhaust pipes for the '73 in excellent condition. Send me a PM if interested.
Also there are no replaceable internal parts or packing for these.

Another thing to keep in mind was that these bikes were LOUD when new compared to modern bikes.  

The "noise control act of 1972" came into being in the USA and motorcycles were identified as a major source of "noise pollution".  Regulations were enacted over the next couple of years to limit sound levels of new bikes.  This is one of the reasons Yamaha had to stop selling the DT400 here in 1978.  Over the years, those limits have been lowered over time.

Bottom line is the "unregulated sound level" of a 1973 motorcycle like an RT3 was literally over twice as loud and as much as four times as loud as a more modern bike!  Huge difference even if the 1973 RT3 was brand new! 

(Modern street legal bike standard is 70 dBa.  RT3 was pushing about 86-88 dBa when new.  Sound level doubles going from 70 to 80 dBa and quadruples going from 70 to 90 dBa.)    

Greetings from Tennessee 

"Another thing to keep in mind was that these bikes were LOUD when new compared to modern bikes."

Agreed! So there might not be anything wrong with the exhaust.

My 1972 R5 is noticeably louder than my 1973 onwards RD250/350/400s.

 

If it's an RT3 as in 1973 & you look deep in the end you should see the spark arrestor.
If its a DT360A 1974 you should still see that down in the removable end muffler.
If it's been removed it'll sure be loud.
It's only spot welded in & we used to knock them in off the welds then shake them back out on some RT2's & 3's when they got severely carboned up, but nowdays i'd leave it in & burn the carbon off.
On my old rat bike 250 i welded something on the end that's got a baffle of sorts in there.
Pic of spark arrestor below--not mine just a random google long ago or was from someone on this site.
photos.app.goo.gl/P5gey7e1FkpmNPLj6
Found a pic of my poor old DT1 but same basic exhaust as a "73 360 [RT3 that is not DT360A].
Is pretty quiet. The piston rattle & inlet noise is louder haha.
photos.app.goo.gl/3drqYvBBfpqJ2cQb7
Ok it's Uuuggly--but been like it for maybe 40 years.
Thought i'd better add a pic of the left side seeing i mentioned inlet noise--open filter--& the old piston port motors suck loud.
photos.app.goo.gl/oUawcPQzry6wj5Rm7

Thanks. You are correct. The bike is 1973. Sent you PM. 
Best Regards

Duly noted. However, this is like expansion chamber with no silencer loud. 

Thanks for the info. Yeah, there is no spark arrestor in the baffle. Perhaps that does not help with it being obnoxiously loud