(3)  
Check for misadjusted ram air exhaust louvers (Ref 21-51-91).

(4)  
Check for separation of ram air inlet door seals (Ref 21-51-41).

(5)  
Check for misadjusted main landing gear wing door (Ref 32-13-21).

(6)  
Check for delamination or deterioration of trailing edge aftflap (Ref SRM 51-40-6).

C.  Vibration occurs with trailing flaps. Extended between 2 and 5 units and is most noticeable in wing root area. Check the following:
(1)
Check for vertical play in trailing edge flap assembly foreflap (Ref 27-51-41 I/C).

(2)  
Check for wear of foreflap sequencing carriage bearing (Ref 27-51-111 I/C).

(3)  
Check for loose attachment bolts on hinged flap carriage rollers (Ref 27-51-51 R/I).

(4)  
Check for play in trailing edge inboard flap aft segment.

(5)  
Check for wear or deterioration of aft segment actuating mechanism cam roller bearing.

(6)  
Check for misadjusted upper wing trailing edge panels (Ref 27-51-72 A/T).

(7)  
Check for foreflap/spoiler interface blade seals (Ref 27-61-11 R/I).

(8)  
Check for loose leading edge slat actuator access panel.

(9)  
Check for deterioration of trailing edge aftflaps or foreflap.

D.  Vibration associated with noise. Check the following:
(1)  
Check for missing sealant at wing-to-body fairing.

(2)  
Check for discrepant ground service check swing valve (Ref 21-21-11 R/I).

(3)  
Check for loose or vibrating air distribution duct (Ref 21-21-0 thru 21-24-0).

(4)  
Check for misadjusted nose landing gear door (Ref 32-33-11).

(5)  
High pitched noise that can be heard at the aft two to three seat rows.

(a) Check the outflow valve diffuser screen per Service Letter 737-SL-21-30.
E.  Vibration felt in control column.
(1)  Check elevator control cable tension (Ref 27-09-111).
F.  High frequency vibration and sound occurs when the flaps are extended. Vibration can be heard over the wing.
(1)  Check the inboard midflap seal plate (Ref MM 27-51-341).
NOTE:  A misadjusted seal plate can cause a humming noise that can be heard at the seats just aft of the wing.
3.  In-flight Airframe Vibration
A.  Boeing has compiled vibration information obtained from flight test, as well as individual operators. This information was used to create the troubleshooting guide in Figure 201. It is suggested that operators use applicable guides while troubleshooting airframe vibration.
4.  Vibration Diagnosis Flight Test
A.  Fly aircraft at the same speed, altitude, payload, and angle of attack associated with the vibration report. Monitor for onset of vibration. Proceed per “vibration absent” or “vibration present.”
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5-51-281 Page 202  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 01/00 

(1)  "Vibration absent" try to induce vibration per the following procedure. Restore aircraft to normal flight between each step. Step 1. Vary speed (+/-30 knots), altitude (+/-1000 ft.) and angle of attack. Step 2. Apply small up-stabilizer angle and hold; then, apply small down-stabilizer angle and
hold. Step 3. Apply small right-rudder angle and hold; then, apply small left-rudder angle and hold.
(a)  If no vibration is found, terminate procedure and monitor for vibration during service. If vibration cannot be duplicated during troubleshooting, but continues to occur intermittently during service, it is suggested that flight crew use Attachment I to record details of the event. Alternately, flight crews can use Attachment II, at their discretion and the discretion of their flight operations department, to gather more detailed information to assist troubleshooting.
(2)  "Vibration present" - Isolate the vibration location and frequency. A "low frequency" can be felt or seen: up to about 20 hertz; a "high frequency" can be sensed and usually heard above 25 hertz. For the low frequency, try to identify the direction (vertical, lateral, etc.). Then, execute the flight procedures listed on Attachment II and observe/record the effect of each step on the vibration. Small changes may be meaningful. Often a visual observation can confirm movement of an engine or other surface. Allow sufficient time during each step for the vibration to change (30 seconds to 1 minute on condition).
(a)  
If no source of vibration can be identified or isolated to a system, terminate the procedure and monitor aircraft for vibration during service. Contact Boeing Service Engineering for further assistance.

(b)  
　
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