My volvo s40 has developed a higher than usual pitch - it is due a service?

NEED MROE DETAILS! START with an ENGIEN BAY cleaning using GUNK ORANGE ORIGINAL engine cleaner at a local DIY car wash! I tend to think you are having an issue with a SERPENTINE BELT PULLEY! OPEN the HOOD and see if any PULLEYS are WOBBLING!  MIGHT be ALTERNATOR PULLEY! You did NOT tell MILEAGE or more details!<<<When it comes to EUROPEAN based cars, YOU MUST USE ONLY PROPER EUROPEAN FULLY SYNTHETIC ENGINE OIL ONLY! My FAVORTIE is PENNZOIL ULTRA EURO oil cuz it is made from NATURAL GAS and CLEANS as you drive it! IT also has a WARRANTY out to an AMAZING 500,000 miles with only ONE or TWO oil change each year! You REALLY NEED to give MORE DETAILS On this issue!  with PROOF OF USE!>>>>THERE are a FEW RULES To follow here with ANY CAR really! POWER WASH the ENGINE DOWN with GUNK ORANGE at a do-it-yourself (DIY) car wash to remove ALL debris especially BEHIND the RADIATOR Where LEAVES, TWIGS, FETHERS< SEEDS, GRASS< and ANIMAL droppings have accumulated! This will keep your CRANK POSITION SENSOR working and NOT to OVERHEAT! The CPS (really CKS) is an ELECTRO MAGNET that tells the COMPUTER when to fire heh PLUGS and a s SUCH vulnerable to the HEAT of a clogging RADIATOR VANE! >>>>USE ONLY PENNZOIL ULTRA EURO 0W-40.>>>>get a set of DENSO TWIN TIP IRIDIUM LASER spark plugs and INSTALL THEM! IF you EVER used VALVOLINE you will need a DE-SLUDGING TREATMENT to clean out the INSIDE of the engine from BAD HEAT HOLDING TARRY SLUDGE building UP in the OIL PAN< VALVE COVERS< FRONT COVER and SEALS and GASKETS! ONCE AGAIN you MUST use ONLY GENUINE EUROPEAN FULLY SYNTHETIC OIL or things will get WORSE!  Other oils are AMSOIL for EURO cars, TOTAL ENERGY OIL for EURO CARS, MOBIL ONE 0W-40, LIQUI MOLY< MOTUL, RAVENOL>  But only PENNZOIL ULTRA EURO DARES to give a warranty to 500,000 MILES! BE SURE you use ONLY GENUINE VOLVO coolant as well ! NO GREEN PRESTONE or any OTHER type! THE DENSO plugs REACT with our modern LOW SULFUR EUROPEAN type FUELS to give the MOST POWER and FUEL MILEAGE and LOWEST emissions! >>>You MAY need a new WATER PUMP< so check the COOLANT LEVEL to see if it is DROPPIING!  I am ASSUMING you are from another country cuz you type in BROKEN ENGLISH!  Check for WOBBLY pulleys under the HOOD! It si VERY important to keep teh ENGEIN BAY as CLEAN as possible so HEAT will DISSPIPATE most evenly! The PENNZOIL will EVENTAULLY remove YEARS of BAKED IN CRAPPY deposits made by CHEAP oil changes! IF you can find a can of AMSOIL POWER FOAM< use it to clean your THROTTLE BODY and the IDLE AIR CONTROL VALVE found at the OTHER end of the AIR INTKAE SNORKEL from the AIR FILTER BOX!  Without more details.  THIS IS AS far AS i can go! Denso TT IRIDIUM PLUGS ARE TRULY amazing AND WILL always IMPROVE MILEAGE AND POWER! NEVER EVER use ELCHEAPO bargain OIL CHNAGES as they are INFERIOR in every way! Get an oil filter from HENGST if you can FIND it online! They are made from SHEEP FLEECE inside an NOT PAPER like CHEAP filters are made! WHEN YOU ARE NEAR 100,000 MILES>>>>>ALL your fluids MUST be CAHNGED! The BRAKE fluid, the COOLANT< the STEERING fluid, and TRANS fluid! You SHOULD go to ANY EUROPEAN car dealership like AUDI or even VW or of course VOLVO! USE only genuine PENTOSIN FLUIDS from Germany!You ALSO CHANGE the TIMING BELT aty this time if it HAS ONE< and probably DOES! Use 1-800 ALL_EURO in BERKELY< MICHIGAN for PARTS and wear items! GOOD LUCK and I do NOT know whay HGHIER than USUAL PITCH is supposed to MEAN! SO I AM GUESSING HERE a LOT! BYE NOW!   

Higher than usual "pitch"? Like the rear struts collapsed or there's a weird noise under the hood?

Your ears have noted a difference in the way the engine sounds.  That means something.  It can't hurt to take it to a Volvo mechanic for him to listen.  He would be more knowledgeable as to what the sound is.  It could be important (costing you a few dollars) or NOT, costing you 0 dollars.  At least you know.

See a mechanic.  Or just open the hood yourself and observe where the high pitch sound is coming from when the engine is running.  Could be a bad serpentine belt.

Without looking at it? have it checked by a mechanic.

You have a car that sings? Wow you could make money from that. Seriously you should know if a service is due according to mileage or time. No one can comment definitively as we don't know what you mean by 'pitch'.